Yeah I definitely think the idea of grouping units into armies, and then having support costs rise as they get bigger is the way to go.
I actually didn't have in mind gold though. I was thinking that directly as more units are piled into the same tile, the strength of what the unit would be suffers a greater and greater tax. A unit 5 horse archers would have 30 strength, but a stack 20 horse archers would maybe only have 70 strength.
Intuitively you could think of this tax reflecting the problem of "overcrowding", or you could view it as a lot of units would be more likely to run out of supplies, and hungry troops fight poorly.
I'm not going to post in that thread though. If I start talking to civ V fanboys, things are going to get ugly
I mean there's someone in that thread that literally seems upset at the notion that having a bigger economy leading to a bigger army would determine the outcome of a war.....