The current game model, RTW in general, not just EB, ensures that I'll leave have to worry about money, however, I could see the concept of a tree of reserves. Build this building costing 6k to store 5k worth of reserves, gives a 5% happiness bonus. 12k to build a 10k reserve and provides 10%. Beyond that no order bonus, just a way to represent the storage of massive amounts of money. Each time the AI loots one of yours, they get that building, and it's damaged 100% like the gov building. Vary the money as needed, though I would hope the largest one would store a huge amount of gold as can be accumlated by focusing on peace, not war. Build time... not too long, even for the longest one. Anyways, just an idea.