Mod: Pax Malazica.

[HoneyBadger]

In that case, maybe compromise at a 50% increase in cost? That still would give you a gross 25% upkeep, without making them prohibitively expensive.

I don't think there's a resource-upkeep in the game, but it would be kind of fun if there were-maybe reduce Prot (armour) and Move (calvalry) for certain units, over time, that were out of areas with proper infrastructure to maintain their equipment.
Along the lines of Starvation in areas without enough Supply.