Greetings fellow PONers ^^
A few days ago I was looking for details on Army/Navy maintenance cost per turn, and couldn't find any answer on the forums or documentation, so on Pocus kind direction, I looked directly in game files and here is the formula I think is correct. I'm sharing this in case some one else wonders too.
Maintenance cost per turn = Unit initial cost x 0.2%
This is lowered by 10% per level of entrenchment.
The curious or modder can find these informations in Settings/GameLogic.opt file, line 305-307:
milPercUnitUpkeep = 1 // % of each Unit cost that will be paid as "maintenance" upkeep
milMilitaryUnitUpkeepGobalCoeffH = 20 // then multiplied further by this coefficient (x0.2)
milTrenchReductionUpkeep = 10 // each trench level reduces by 10% upkeep cost (garrisoning & wintering)