Well we know upkeep can be reduced if food is supplied from an external source, like hunting. So farming would make sense, and under that idea farmed animals would have monetary value and could be traded. And they would replenish over time, as would wood, vegetables, etc.
And from that, maybe building is cheaper when you have the necessary resources, so upgrading a city could cost less if there is a cache of building materials available.
I was going for a "get wood for sheep" joke, but I sorta forgot how I planned on getting there.
Gentlemen, I like war.
I like trench war, I like Blitzkrieg, I like the offensive, I like the defensive.
I truly love each and every kind of war man can wage on a tabletop game.