GURPS Mass Combat Troop Strength Calculation

GURPS Mass Combat Troop Strength Calculation

During the GURPS-based Braunstein game that I ran earlier this year, GURPS Mass Combat got plenty of action. Since I wasn't satisfied with the selection of troop elements from the book, I opted for a shoddily made conversion formula based on ACKS Domains at War. While it like a decent enough way to approximate Troops Strength (TS) at first, it then became apparent that it was quite awful, and this caused some major issues. The issues also had snowballed into the demise of the game, because lots of factors were based on TS, such as wages, etc. It became clear that the bulk of "prep" for the next installment would be reworking the TS formula from the ground up. The formula had to be grounded in GURPS and not ACKS, as the translation step between the systems messed everything up. It took myself and AlexPsi more than two months to come up with a solution that provided surprisingly sensible results (what's even more surprising, that the results were also in some aspects quite similar to those in GURPS Mass Combat and ACKS). It's time to describe the approach in detail. Be warned - this is not necessary for your regular GURPS game. This is for those people who like to overcomplicate stuff (so, like a half of the GURPS community, or, in other words, ~5 people).

SPREADSHEET LINK (view only - make a copy for yourself if you want to use it)

"But Eggplant, why are you reinventing the wheel? Why don't you just use Heroes on the Mass Scale from Pyramid #3-84?" First, it's intended for heroes and not generic troops. Second, I don't think the author has ever run a mass combat.

Anyway, one of the issues is the additive nature of the approach from the aforementioned article. If you get better defense, you become able to dish out more attacks before collapsing, and hence become better at offense. Thus, this is clearly multiplication. (Although, there are some aspects that seem that they'd call for addition, but I will mention them later, unless I completely forget about them.) What do you multiply? Here's where I, in my attempt to create an ACKS-less formula for GURPS, came back to ACKS, because the way it does that seems very good to me.

Essentially, individual Troop Strength (iTS) = OffensiveFactor * DefensiveFactor * MovementFactor * MoraleFactor * SpecialAbilityMod / 13.859.

The first question you might ask is "What is individual Troop Strength?" In GURPS Mass Combat, TS is per element, which usually is ~10 men, ~5 mounted riders, ~1 large monster or siege engine, etc. Here, I decided to make it more generic and calculate TS on the per-troop/figure basis. It's still very easy to create prebuilt "elements" of any scale, such as 10/60/120 light infantry or anything else.

Now, before I explain how to calculate these factors, I have to address one of the biggest points of contention about GURPS Mass Combat. Based on the experience of myself and my players, it's been found out that TS should be split into separate values for separate special classes. Thus, whereas before you'd have an element of bowmen that would provide TS 1 and have the Fire special class, how you can have an archer that has iTS 0.332 and Fire TS 1. Thus, iTS is used to calculate the TS superiority bonus and casualties, while special class TS only matters for special class superiority. This is done to preserve the ability to customize troops but avoid situations such as giving a very powerful melee fighter a lousy bow that he can use at skill 8, and the fighter suddenly performing superbly well in mass combat as a Fire element. This was a big problem in the aforementioned Braunstein game. It might seem like it would make calculation of special class superiority more difficult, but that's not true - you're still adding up the same number of numbers, it's just that the numbers are different now.

Speaking of special classes, not all of them have been separated to have their own TS values.

Special classes with their own TS values: Artillery, C3I, Engineering, Fire, Recon.

Special classes without their own TS values (thus, working as before): Air, Cavalry, Naval.

The latter three don't have their own TS values because there is no way to gauge the degree of aptitide. You're either are a flier or you are not. There's no "this guy can fly fly-ier than the other flier".

You also might've noticed that that Armor special class has been deprecated. The defensive qualities are already baked into iTS, and the Armor class is only used for the superiority bonus in GURPS Mass Combat - it does not interact with anything else (such as strategies, where other classes may be important). Thus, it's not really necessary. Artillery has also been reworked, but that'll be explained later.

So, now it's time to uncover the math behind the calculations. Please, do not try to calculate it without a spreadsheet - you'll go insane. I will link a spreadsheet that makes the calculation relatively painless.

1. OffensiveFactor

This one is the most difficult one, as it is actually a composite factor of three other factors - MeleeFactor, RangedFactor, and ArtilleryFactor.

If MeleeFactor > RangedFactor, then OffensiveFactor = MeleeFactor + RangedFactor/5

If MeleeFactor <= RangedFactor, then OffensiveFactor = RangedFactor + MeleeFactor/20

Round up to three decimal numbers.

MeleeFactor = MSkillMod * MAvgDamageMod * MDamageTypeMod * UnreadyMod * SizeMod

MSkillMod is a bit complicated. Take the effective skill level for the best melee attack of the creature. By "best" I mean the one that gives you the highest MeleeFactor. If you have Extra Attack without Multi-Strike, then add +4 to your effective skill per level of Extra Attack. If you have Extra Attack with Multi-Strike (or ATR), add +5 to your effective skill per level of Extra Attack. If the resulting effective skill is 3-15, simply look up your probability of success on p. B171. If it's higher, then add +5% per each level past 15. For example, skill level 13 would result in MSkillMod = 0.838 (83.8% chance of success), while skill level 18 would result in MSkillMod = 1.104 (110.4% chance of success, i.e. 95.4% + 2 * 5%).

What's the reasoning behind all this? Skill past 15 doesn't really improve your chance of success, as you either hit or roll a critical failure. However, the higher skill is still useful, as it allows you to target hit locations that are more difficult to hit, be better at Deceptive Attacks and Feints, etc. Thus, this +5% increase is a simple, abstract measure of that. Extra Attack does not double MeleeFactor, as that would result in some ridiculous values, but effectively adds to your skill. What is Extra Attack if not +6 to your skill with a Rapid Strike? Here you get +5 because you do not benefit from better Feints and all the other stuff I mentioned above, or +4 because you also have to use a worse weapon for the second attack. How much worse it is doesn't matter - it's below the level of resolution of this system.

MAvgDamageMod is the average damage of your best melee attack. Thus, if you deal 1d damage, you input 3.5.

MDamageTypeMod is 1.2 for cutting, impaling, large piercing, huge piercing, corrosion, fatigue; it is 1 for everything else.

UnreadyMod is 1 or 0.8, if your weapon becomes unready after the attack. Why 0.8 instead of 0.5? Because you're likely to be fighting alongside allies who'd help you mitigate this disadvantage, and you're also likely to have a backup weapon.

SizeMod is 1 + SM/10. A lot of ink has been spilled on arguments about whether high SM is an advantage or a disadvantage in GURPS. Here, it is an advantage because you get higher Reach and, most importantly, can trample and overrun smaller opponents and are less likely to be trampled/overrun yourself. Trampling is actually a huge deal. We've tried the formula without SizeMod, but found that including it as an advantage provides more reasonable values.

Now, why is the formula different based on whether ranged or melee attacks are better? I shamelessly stole it from ACKS Domains at War. It's not explained there, but I assume that it is because if you're a good melee fighter, a ranged attack is a nice bonus, but it's not crucial - you're preferring melee combat. If you're better at ranged combat, then your melee attack is only a minor bonus, as you're actively trying to stay outside melee, so you're not actually going to be using it much.

You want to get nuts? C'mon, let's get nuts!

If ArtlessRangedFactor > ArtFactor, then RangedFactor = ArtlessRangedFactor + ArtFactor/5

If ArtlessRangedFactor <= ArtFactor, then RangedFactor = ArtFactor + ArtlessRangedFactor/5

Round up to three decimal numbers.

Now, it's time to explain what counts as a ranged attack and what counts as an artillery attack. In this case, a ranged ("art(illery)less") attack is a ranged attack that hits a single target. An artillery attack is an area attack (Area Effect, Cone, or Explosion) even if it's a melee attack/emanation. I know that it doesn't make sense, but you have to deal with me here. Artillery is dumb in GURPS, to put it mildly. You get some rules for it in GURPS Low-Tech and some rules for it in GURPS High-Tech, but nothing really makes much sense. Yes, a mounted arbalest can be angled upward for indirect fire, but you have have ST 15 (which is achievable even in a realistic game) and pick it up... don't you dare angle it upward! What is an arbalest if not a big crossbow? Why can't you use any crossbow for indirect fire? Why can't you use a bow or a sling for that? Why does High-Tech allow cannons, mortars, and machine guns to use indirect fire, but not other guns? What makes machine guns so special? Why do GURPS Mass Combat artillery elements list examples that are not artillery as per GURPS Low-Tech? Why does that Heroes on the Mass Scale article list such requirements for artillery elements that low-tech artillery becomes impossible? See what I mean? To preserve my sanity, I had to reframe "Artillery" as area attacks.

ArtlessRangedFactor = RSkillMod * RAvgDamageMod * RDamageTypeMod * RclMod * RHalfDMod * RReloadMod * RLowAmmoMod / (FP Cost + 1)

RSkillMod is calculated the same way as MSkillMod above, i.e. the skill roll success chance for the best ranged attack. Extra Attack modifies it in the same way. However, here you also have to add your Accuracy and RoF bonus to the effective skill.

RAvgDamageMod is the average damage of your best ranged attack.

RDamageTypeMod is 1.2 for cutting, impaling, large piercing, huge piercing, corrosion, fatigue; it is 1 for everything else.

RclMod is 1.15 for Rcl 1, 1.1 for Rcl 2, 1.05 for Rcl 3, or 1 for Rcl 4+. If you have RoF 1, assume RclMod of 1.

RHalfDMod is based on your attack's 1/2D Range. RHalfDMod = (1/2D Range / 150)^0.5, but no higher than 1.3.

RReloadMod depends on how quickly you can reload. Do not assume Fast-Draw.

     Reload 0-1: RReloadMod = 1.05

     Reload 2: RReloadMod = 1

     Reload 3: RReloadMod = 0.9

     Reload 4: RReloadMod = 0.8

     Reload 5: RReloadMod = 0.75

     Reload 6: RReloadMod = 0.6

     Reload 7: RReloadMod = 0.55

     Reload 8: RReloadMod = 0.5

     Reload 9: RReloadMod = 0.45

     Reload 10-19: RReloadMod = 0.4

     Reload 20-29: RReloadMod = 0.35

     Reload 30-39: RReloadMod = 0.3

     Reload 40-59: RReloadMod = 0.25

     Reload 60+: RReloadMod = 0.2

RLowAmmoMod is 0.5 if you have only 1-3 pieces of ammunition, or 1 otherwise. This is mostly relevant for thrown weapons. However, if the creature's primary melee weapon can be thrown, but the creature is very unlikely to throw it (for example, a spearman with no backup weapon), do not count it as a ranged attack at all. Don't be silly.

Of course, if you have no single-target ranged attack, then your ArtlessRangedFactor is 0.

Likewise, if you have no area attack, then your ArtFactor is 0. If you do have one, use the following formula:

ArtFactor = ASkillMod * AAvgDamageMod * AReloadMod * AAreaMod / (FP Cost + 1)

ASkillMod is calculated the same way as MSkillMod above, i.e. the skill roll success chance for the best ranged attack. Extra Attack modifies it in the same way. However, here you also have to add your Accuracy to the effective skill.

AAvgDamageMod is the average damage of your best area attack.

AReloadMod is the same as RReloadMod above.

AAreaMod depends on whether your area attack is, well, an area attack, cone attack, or an explosion.

1. Area Effect

AAreaMod = AreaMaxRangeMod * RadiusMod * DissipationMod * BombMod * AreaPersistentMod

In this case, AreaMaxRangeMod = (Max Range / 150)^0.5, but no higher than 1.3.

RadiusMod = (Area Radius)^0.5

DissipationMod is 0.5 if your attack has Dissipation, or 1 otherwise.

BombMod is equal to the success chance of the effective skill of your Bombardment limitation, if you have one. If not, it is equal to 1.

AreaPersistentMod = (1 + Area Radius * Persistent Duration / 100), but no higher than 1.6. If your attack does not have Persistent, then this modifier is equal to 1.

2. Cone

AAreaMod = ConeWidthMod * ConeHalfDRangeMod * ConeMaxRangeMod * DissipationMod * BombMod * ConePersistentMod

ConeWidthMod = (Width * Max Range / 15)^0.5

ConeHalfDRangeMod = (1/2D Range / 150)^0.5, but no higher than 1.3.

ConeMaxRangeMod = (Max Range / 150)^0.5, but no higher than 1.3.

BombMod is equal to the success chance of the effective skill of your Bombardment limitation, if you have one. If not, it is equal to 1.

ConePersistentMod = (1 + Width * Persistent Duration / 100), but no higher than 1.6. If your attack does not have Persistent, then this modifier is equal to 1.

3. Explosion

AAreaMod = ExplMaxRangeMod * ExplLevelMod * ExplLowAmmoMod

In this case, ExplMaxRangeMod = (Max Range / 150)^0.5, but no higher than 1.3.

ExplLevelMod depends on the level of your Explosion modifier. If you have Explosion 1, then ExplLevelMod = 0.2828. If you have Explosion 2, then ExplLevelMod = 0.3. If you have Explosion 3, then ExplLevelMod = 0.5657.

ExplLowAmmoMod is 0.5 if you have only 1-3 pieces of ammunition, or 1 otherwise. This is mostly relevant for thrown weapons, like grenades.

In any case, if the attack is an emanation, your effective skill is 15, and max range is equal to radius.

The artillery calculations are the most questionable ones here (especially for cones and explosions), but from whatever examples I ran through it, the values are... good enough. I might adjust the math in the future, but we'll see.

2. DefensiveFactor

This one is much easier.

DefensiveFactor = ActDefMod * HTMod * (HP + 4 * AvgDR) / 4

ActDefMod = ActDefMod1 * ActDefMod2 * ActDefMod3, where ActDefMod1-3 are the success probabilities (just like the skill modifiers for attacks) of the three best active defenses against frontal melee attacks. Thus, if you have Block 12, Parry 11, and Dodge 10, you'll get 0.741 * 0.625 * 0.5. If you have Parry 10 and Dodge 9, you'll calculate it as for values of 10 (parry), 9 (dodge), 8 (penalized second consecutive dodge). In any case, include your shield's DB. Don't forget that two-handed weapons have halved consecutive parry penalties.

HP is self-explanatory, but AvgDR is calculated as follows:

1. Calculate DR for every major hit location and mupliply it by the body coverage percentage: chest (75%), abdomen (25%), skull (20%) (don't forget to include the innate DR 2 here), face (10%), neck (5%), arms (50%), hands (10%), legs (100%), feet (10%). If you have partial coverage, mutiply your DR by the degree of coverage, rounding down. Thus, for things like boots protecting your legs 1 in 6 times, don't even bother including it.

2. Calculate the average value of those weighted DR values and multiply it by 305% (that's the total body coverage).

HTMod is as follows (include modifiers from Fit/Very Fit/Unfit/Very Unfit here).

     HT 0-6: HTMod = 0.4

     HT 7: HTMod = 0.6

     HT 8: HTMod = 0.8

     HT 9: HTMod = 0.9

     HT 10: HTMod = 1

     HT 11: HTMod = 1.1

     HT 12: HTMod = 1.15

     HT 13: HTMod = 1.2

     HT 14: HTMod = 1.25

     HT 15: HTMod = 1.3

     HT 16: HTMod = 1.35

     HT 17: HTMod = 1.35

     HT 18+: HTMod = 1.4

3. MovementFactor

MovementFactor = 1 + Highest Move * FlightMod / 100

Here, use the highest Move in any medium, taking into account things like Enchaned Move and, of course, encumbrance.

FlightMod is 1.5 if you can fly, or 1 otherwise.

In combat, especially mass combat, speed isn't actually that important. Thus, the movement factor isn't a very significant part of the iTS calculation. Flight, however, is important as it allows you to circumvent terrain and other obstacles, and also gives you better defenses (retreating upwards is strong). One of the issues with the initial ACKS-based approximation was that flight inflated the troop strength to some absolutely unreaasonable numbers, which became a problem due to one of the players running a faction of fliers.

The spreadsheet also has the "Sustained Swimming" checkbox, but the only thing it does is give the creature the Naval class. Figure out yourself, what counts as sustained swimming.

4. MoraleFactor

MoraleFactor = (1 + FearMod / 50) * WillMod

FearMod in this case is equal to the modifier to resist Fright Checks from things like Fearlessness, Fearfulness, Cowardice. Don't forget that Combat Reflexes is effectively two levels of Fearlessness for this purpose. If you have Unfazeable, then your FearMod is 10.

WillMod is as follows:

     Will 0-6: WillMod = 0.9

     Will 7: WillMod = 0.925

     Will 8: WillMod = 0.95

     Will 9: WillMod = 0.975

     Will 10: WillMod = 1

     Will 11: WillMod = 1.025

     Will 12: WillMod = 1.05

     Will 13: WillMod = 1.075

     Will 14: WillMod = 1.1

     Will 15: WillMod = 1.125

     Will 16: WillMod = 1.15

     Will 17: WillMod = 1.175

     Will 18+: WillMod = 1.2

As you can see, WillMod is less significant that HTMod. Will doesn't actually play a role in morale in GURPS, but it's included here to represent the resistance against mental afflictions.

5. SpecialAbilityMod

This is something we've been thinking about a lot. GURPS is such a flexible system that it's impossible to take everything into account. What is described above it a very rough approximation. For example, how do you include something like spellcasting? Sure, some spells might be ranged attacks, some may be artillery or melee, but there's a lot of other spells that are useful in combat, but are not reflected in all of these factors. Abilities are myriad, but I strongly recommend abstracting them away into a modifier to the creature's iTS. My suggestion is to treat abilities as minor (+10% to iTS), major (+20% to iTS), or great (+40% to iTS). Disadvantages that affect combat also can be treated as drawbacks - minor (-10% to iTS), major (-20% to iTS), or great (-40% to iTS).

For example, a dark dwarf has Resistant to Poison (+8) is a stong ability, but in terms of combat capabilities, it is rather minor, so I have it give +10% to iTS. A helmet might give No Perhiperal Vision, which is a -10% iTS drawback. A wizard might have an assortment of spells that (aside from the attacking ones, perhaps) collectively count as +40% to iTS. No need to overcomplicate things, but it'll be useful to keep a list of these abilities to preserve consistency.

6. 13.859

Where did this come from? This is the baseline iTS of a spearman with ST 12, skill 12, 1d+1 imp damage, DR 2 on his torso and feet, and a DB 2 shield. Thus, iTS is measured in spearmen. This is something done both for convenience and utility, as it'll also give us the baseline for wage determination.

Special Classes and Special Class TS

As was mentioned before, some special classes now have their own TS values. However, it's not enough to have Stealth to be a Recon element or a ranged attack to be a Fire element.

Fire TS = RangedFactor / 3.995, but only if Max Range is 100 or higher

Thus, only combatants with a long-ranged attack have Fire TS. In any case, the ranged attack does play a role in the iTS calculation, but short-range weapons, such as most thrown weapons, do not give you Fire TS. 3.995 is another baseline based on the RangedFactor of the archer from GURPS Monstrous Compendium.

Artillery TS = ArtFactor / 1.363, but only if Max Range is 100 or higher

Same thing here. A grenade doesn't give you Artillery TS. 1.363 is based on a 1st-level Fireball.

Recon TS = StealthMod * ObservationMod * MovementFactor * (1 + InvisMod + ObscureMod) / 0.7124 / 0.981

    Prerequisites: Stealth 14+, or Invisibility, or at least 9 points in Obscure

StealthMod is the probability of success of the Stealth skill roll.

ObservationMod is the probability of success of the Observation skill roll. Yes, if you don't have the skill, you're heavily penalized.

InvisMod = (point cost of Invisibility^0.5) / 5 / (1 + FP cost)

ObscureMod = (point cost of Obscure^0.5) / 5 / (1 + FP cost)

For both Invisibility and Obscure, you have to use common sense. If it's something like Obscure (Taste), it doesn't matter for Recon. If it's Invisiblity (Only against animals), it also doesn't count for Recon.

As for 0.7124 / 0.981, I... don't remember. Essentially, that should be the baseline for a Stealth-14 and Observation-12 scout, but I don't remember why there are two numbers there. It doesn't really matter.

Engineering TS = EngSkillMod * (1 + InsubMod + PermMod + TunnelMod) / 0.907

There are no clear prerequisites here, but you either need to have an appropriate skill and equipment, or one of the abilities that will be detailed below.

EngSkillMod is the probability of success for the highest appropriate engineering skill. The skills that could be used include Explosives (Demolition), Engineer (Combat), Engineer (Civil), but you can include anything else that you find fitting, something like Liquid Projector (Flamethrower).

InsubMod = (point cost of Insubstantiality^0.5) / 10 / (1 + FP cost)

PermMod = (point cost of Permeation^0.5) / 10 / (1 + FP cost)

TunnelMod = (point cost of Tunneling^0.5) / 5 / (1 + FP cost)

You can see that these three are rather similar, but Insubstantiality and Permeation are worth less than Tunneling. This is because a tunnel can be made to allow other troops to circumvent fortifications, but Insubstantiality and Permeation are personal - they can be used to, for example, phase through the castle walls and open the gates from inside. As always, use common sense - if you have Permeation (Paper), you don't become an Engineering element.

C3I TS = C3ISkillMod * (1 + ClairsentienceMod + PrecogMod + TelecomMod) / 0.907

C3ISkillMod is the probability of success for the higher of IQ and Intelligence Analysis skill.

ClairsentienceMod = (point cost of Clairsentience^0.5) / 5 / (1 + FP cost)

PrecogMod = (point cost of Precognition^0.5) / 5 / (1 + FP cost)

TelecomMod = (point cost of Telecommunication^0.5) / 5 / (1 + FP cost)

Here, Clairsentience and Telecommunication must have range of at least 1,000 yards to count. Precognition must be usable for battle purposes. Again, use your own judgment here.

Finally, we have the three TS-less special classes:

To be a Naval element, you need to have a way of sustained swimming.

To be an Air element, you need to be able to fly.

To be a Cavalry element, you need to have Move higher than 10 and not have Flight. Air and Cavalry are two mutually exclusive classes.

SUPPLIES

The spreadsheet has another tab - Supplies. This one is used to calculate how much food and water does your creature need per day or any other number of days. In terms of values that you have to input, it's rather self-explanatory - SM, Cold-Blooded, Increased Consumption, Reduced Consumption, Cast-Iron Stomach. There's also the Carnivore checkbox that triples the costs, and an arbitrary modifier that you can introduce.

Food weights are taken from GURPS Bio-Tech. This table assumes trail rations - they weigh less than regular food, cost more, and have a longer shelf life. I already wrote a post with my ramblings about food being surprisingly confusing and incomplete in GURPS.

COSTS AND TIMES

Of course, iTS and special class TS aren't the only values that we need. Most importantly, we need to find the monthly wage and cost to maintain. Less importantly, we need to find costs and times to raise the creature, and prices for it as a slave (don't tell SJGames).

Monthly Wage = $350 * (iTS + Fire TS + Recon TS + Engineering TS + Artillery TS + C3I TS) * WageAbilityMod * WageSkillMod * FlightMod

I'll remind you that Cost of Living for a Status 0 creature is $600 regardless of TL. An average wage on TL4 is $700, and it's only marginally lower at TL3. GURPS Banestorm says that a mercenary should earn ~$700 per month, while GURPS Fantasy says that an archer earns ~$350 per month. It should also be noted that soldiers and mercenaries expect a share of the spoils of war, so their actual income is higher (if they survive). I tried using $700 as the baseline, but it provided numbers that are too high. Thus, $350 was chosen, and we ended up with numbers that are more reasonable. For certain troops, you can argue that they are not realistic, but I don't care.

WageAbilityMod is another somewhat arbitrary modifier. Remember SpecialAbilityMod for iTS? That one affects iTS and hence affects the wage as well. However, there are plenty of abilities that do not affect combat capabilities, but are nonetheless useful outside of combat, which can be highly important on the strategic level. For example, a succubus might be able to polymorph into any humanoid being. In combat, that's useless (just like the succubus herself), but outside combat, this is an extremely powerful ability. Thus, this additional modifier uses the same minor (+10%/-10%), major (+20%/-20%), and great (+40%/-40%) approach, but only for the wage. One important case that should be mentioned is any ability that lets you see in the dark well. This is +20% to wage and gives the Night feature to your creature, which is consistent with GURPS Mass Combat.

WageSkillMod is another modifier that only inflates the wage. This is equal to 1 + 0.05 for each skill level above 10 for each skill that wasn't used in any other calculations. Thus, this includes all the various knowledge, maintenance, athletic, social skills, etc. Stock troops are unlikely to have many of them (likely candidates are skills like Soldier, Fast-Draw (Arrow), unarmed combat skills, and weapon skills for non-primary weapons). These creatures spent (or wasted) their time and effort learning these skills, and some of them might indeed be very useful. This is less important for armies, but becomes much more important for regular hirelings.

FlightMod is the same 1.5 multiplier if you have Flight that is used in MovementFactor. Flight barely factors in iTS calculation, but flying troops definitely should be more costly. Flight is a huge strategic advantage.

Finally, Monthly Wage cannot be lower than Minimum Wage.

Minimum Wage = $140 * Daily Food Cost / 3

Daily Food Cost is derived from the Supplies tab. Essentially, this is Poor-tier wage multiplied by how much more expensive or cheap food is for you compared to a regular human. Thus, for human troops this is merely $140. This means that the wage must be high enough to be able to support yourself at Status -2/Poor wealth.

Next up, we need to derive Cost to Maintain, which is an important statistic from GURPS Mass Combat. This includes supplying the creature with food, water, ammunition, wage, maintaining it in the field... wait, wage? Yes, it does make sense that wage is included, but I think it makes less sense that this means that the main purpose of the Logistic Force is to haul money, not actual supplies. I believe that it'd make sense to keep Cost to Maintain as something you have to pay once a month, and also derive the Supply value that defines how much supplies you require and how much Logistic Force is required to supply you.

Cost to Maintain = Daily Food Cost * 45 + Monthly Wage

Why 45? That's 30 days of food with a x1.5 multiplier to abstractly account for water, ammo, maintenance, pack animals, porters, etc.

Supply = Daily Food Cost * 45

To calculate how much Logistic Force (LS) you need to maintain the creature, divide Supply by 1,000. Thus, a regular human soldier requires $270 of supplies, which requires LS 0.27.

Time to Raise is calculated from Cost to Raise as per GURPS Mass Combat, but we'll change things up here. Assuming trainers with Teaching-15 and intensive training (and ignoring HT/Will rolls and some other fiddly modifiers from GURPS Social Engineering: Back to School), we get 1 character point per week of training that costs $70. As per GURPS Mass Combat, we also need to maintain the creature during the training period, but I believe that it shouldn't be paid its full wage during training.

Time to Raise (in weeks) = # of character points in learned skills/advantages

Cost to Raise = Equipment Cost + $70 * Time to Raise + (lower of Monthly Wage and 5 * Minimum Wage) * Time to Raise / 4

Surprisingly, the resulting values usually are quite sensible and close enough to GURPS Mass Combat and ACKS.

Finally, we need the slave prices. In ACKS, we have Untrained Cost and Trained Cost, so I decided to go with it here as well, because it actually meshes well with GURPS. Fortunately, despite the recent developments, GURPS has a slave-pricing scheme in the Monster Slavers article from Pyramid #3-47. That article separates slaves into categories - laborer slaves, soldier/gladiator slaves, and pleasure slaves.

Untrained Cost is the higher of laborer slave price, pleasure slave price, and Body Cost. I'll skip the formula description here, because you can either read the article or use the spreadsheet, where it's rather intuitive.

Body Cost is the value of all the meat, skin, bones, feathers, special components that you can gain by butchering the creature. The spreadsheet has a formula based on the butchering rules from GURPS Low-Tech Companion 3

Trained Cost is the higher of laborer slave price, soldier/gladiator slave price, and pleasure slave price. Speaking of slave troops, the Supply value we calculated comes in handy, because you don't have to pay them wages, so Supply essentially becomes Cost to Maintain for slave troops.

The aforementioned approach to slavery works for sapient creatures, but not for animals. For animals, use the following formulas:

Untrained Value = Body Cost

Trained Value = Cost to Raise + the higher of Body Cost and ($325 + 15 * Basic Lift)

(That latter part comes from the equine pricing from Nordlond Ovinabokin.

Time to Raise can be looked up on p. B459 for general training. The same page says that you should add one year for war-trained mounts.

RESULT

Now, the Capturing and Training and Mass Combat Statistics blocks in my Monstrous Compendiums will be (and some already are) rewritten. They have the following format now: 

    Example #1 - Dark Dwarf Gunner

    Capturing and Training

    Untrained Value: $10,200.

    Trained Value: $12,240.

    Supplies: $2.25 and 0.55 lbs./day of food and 0.5 quarts (1 lb.) of water.

    Wage (Monthly): $1,330.

    Mass Combat Statistics

    iTS: 1.946.

    Fire TS: 0.484.

    Special Classes: None.

    Features: Night.

    Move (Daily/Weekly, miles): 12/48.

    Cost/Time to Raise: $11,510/42 weeks.

    Cost to Maintain: $1,440.

    Supply Cost: $110 (LS 0.11).

    Notes: Infravision is a major ability (+20% to wage); Resistant to Poison (+8) is a minor ability (+10% to iTS).

    Example #2 - Tiger

    Capturing and Training

    Untrained Value: $4,340.

    Trained Value: $52,700 (general) or $101,060 (war mount).

    Supplies: $40.8 and 3.4 lbs./day of food and 6.8 quarts (13.6 lbs.) of water. If it is allowed to hunt, then supply cost is $0.

    Training Skill: Animal Handling (Big Cats).

    Riding Skill: Riding (Big Cats).

    Wage (Monthly): $1,910.

    Mass Combat Statistics

    iTS: 2.749.

    Special Classes: Cv.

    Features: Night.

    Move (Daily/Weekly, miles): 48/192.

    Cost/Time to Raise: $48,360/48 weeks (general) or $96,720/96 weeks (war mount).

    Cost to Maintain: $3,750.

    Supply Cost: $1,840 (LS 1.84).

    Notes: Night Vision is a major ability (+20% to wage).

FINAL THOUGHTS

Of course, this is not perfect. I don't think it's possible to take everything into account, but that wasn't the intention here. You definitely can game the system by, for example, inflating iTS by giving your melee troops cheap crossbows with 1 or even 0 points in the skill or giving archers large DB 3 shields that they are intended to use in melee combat. If something looks like an unreasonable way to abuse the system, tell the person to kick rocks.

Can this be used outside low-TL games? Probably, but some numbers will need adjustments, such as range requirements for special classes and TS baselines.

Can this be used as Challenge Rating? No idea, but if you do want to give it a shot (why though?), it'll probably perform better than CER.

But you know what's even better? Now you know how much money a creature is supposed to earn based on its capabilities, so you can use it to derive its "virtual" Wealth and devise something like Treasure Type for GURPS. Isn't that great?