The XML for the game has a few entries for unit maintenance.



One is a "gold weight per unit divided by 100" which is set to 50, or 50%. This might explain why only every other unit increases maintenance, due to some rounding error.



There is also a "unit maintenance exponent" which is 1.01.



My guess would be the formula is going to be something like



units * 0.5 * constant * ( turns * some fractional constant ^ 1.01)



Not exactly that formula, but the gist of it I'm guessing, where every second unit increases maintenance by an amount that grows as the turn count increases (aka. inflation).

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