Short form: Upkeep costs for military units are nonsensical. Not only are they most definitively NOT the costs listed in-game for the units, but they skyrocket as the number of units increases. Here are two examples, one a Tank unit and the other a Trooper (infantry) unit, both belonging to the CHCH-T.

Fusion Cannon upkeep
s/be 2 credits/turn/unit
1 tank = 2: 2/ea
2 tanks = 8: 4/ea
3 tanks = 18: 6/ea
4 tanks = 32: 8/ea
5 tanks = 50: 10/ea
6 tanks = 72: 12/ea

Battle Trooper upkeep
s/be 2 credits/turn/unit
1 trooper = 1: 1/ea
2 troopers = 4: 2/ea
3 troopers = 9: 3/ea
4 troopers = 16: 4/ea
5 troopers = 25: 5/ea
6 troopers = 36: 6/ea
7 troopers = 49: 7/ea
8 troopers = 64: 8/ea

In the case of the Fusion Cannon, the cost per unit is number of units squared times cost of one unit.
In the case of the Battle Trooper, the first unit doesn't even cost what it should; it costs half. It would be the same formula as the Fusion Cannon if it didn't get the per unit cost wrong. Instead it's just number of units squared.

Not only is none of it what the game says it should be, but it's not even consistent from one unit to another.

I'll grant that racial variations (strengths and weaknesses) could affect this.. but the CHCH-T don't have one relating to unit upkeep, not of all units, nor of tanks nor troopers individually.

Some have suggested that it's increased by the necessity to ship food, for example, but all these measurements took place with the units in one of my territories that overproduces food by more than enough to support them. There's no indication of a shipping surcharge of any kind.

It's also not a penalty for stockpiling too many troops in one territory. Some of these measurements were duplicated with the units divided between two of my territories.

Is this WAI? There's nothing in the User Manual nor in-game that suggests it should work like this.

More observations:

Best I can tell, they subdivide into categories and group similar units by category for this calculation. I expect that observation will determine that the categories are troops, tanks, ships, and aircraft.

- ignores items with no upkeep.. mostly
- Colonizer counts as a tank and Sea Colonizer counts as a ship, even though they have no upkeep cost. I seem to have been charged 3 CR for my sole Sea Colonizer when I had no other ships, and 2 CR for my Colonizer.

It gets weirder. When I had infantry including:
1 Battle Trooper (2 credits)
2 Commanders (1 credit)
1 Medic (1 credit)
1 Scout (0 credits)

I got charged 16 credits for upkeep. Clearly the Scout is discarded (0 upkeep) and the Battle Trooper upkeep is still calculated at 1 CR. (4 units ^ 2 = 16 * 1 upkeep = 16 upkeep).

Another example. The turn review specifies 4 ships:
Attack Sub (2 CR)
Siege Cruiser (5 CR)
Attack Submarine (2 CR)
only remaining ship Sea Colonizer (0 CR)

and the total upkeep for the four was 48 CR. Tell me the math on that one. 4 units ^ 2 = 16; 48 / 16 = 3. The Sea Colonizer is treated as though it has an upkeep of 3 instead of the zero it claims: Disbanded all my ships except the Sea Colonizer and got charged 3 CR in upkeep for 1 ship. Which does clarify the logic. It adds their upkeeps (5 + 2 + 2 + 3 = 12 CR) divides by the number of ships (12 / 4 = 3 CR upkeep/ship), then multiplies that by the number of units squared. No idea how rounding is handled since this one works out with no remainder.

Final example. I was charged 64 upkeep for 8 troopers. Here are the infantry I had at the time:
5 Battle Troopers (2 CR) - consistently treats as 1 CR
2 Commanders (1 CR)
Medic (1 CR)
Scout (0 CR)

That's a total of 9 Troopers (infantry). Clearly in this case, we discard the Scout (0 upkeep), leaving us with 8 valid Troopers. 8 ^ 2 = 64, so it treated all the Troopers as though they had a 1 CR upkeep cost.. at least the Battle Trooper error is consistent.

There's clearly some errors. I'm not happy with squaring the total number of units of each type and then multiplying that by the average upkeep of the units (not sure about rounding), but that seems to be the math.

The Colonizer (2 CR) and Sea Colonizer (3 CR) are not charged the 0 CR in upkeep that the game claims.
And the Battle Trooper is clearly charged 1 CR in upkeep instead of the 2 CR it claims.
No idea how many more such errors there may be.

But in any event, with a quadratic growth pattern in unit upkeep costs, it quickly becomes prohibitive to have an army larger than perhaps a dozen units of one type. Upkeep is somewhat better if you have a mixed army, since it squares the number of each type individually instead of collectively, so you want to have a mixture of tanks, troopers, and aircraft when you attack (ships don't really mix well with the others). (If, for example, you had 3 troopers, 3 tanks, and 3 aircraft, if they were squared collectively that'd be 9^2 = 81 * their average upkeep. Individually it's 9 * avg upkeep + 9 * avg upkeep + 9 * avg upkeep = 27 * avg upkeep.. 27 * upkeep is considerably less than 81 * upkeep.) [All this is simplified as if the avg upkeep of all units were the same for purposes of making the argument easier to follow.]

What this means is that if you have a technological deficit in military units, the 'zerg rush' concept of burying your enemy in your superior numbers is not feasible. You need to stay at or near technological parity with your opponents, or the cost of maintaining a credible military balance quickly becomes prohibitive. This does not make me happy.