Add a building that is used to store coins and some other things that can be either made with a blacksmith like golden statues and etc also like gems from a jewelcrafter rings and such. It can work on a simple system since EVERYONE LOVES PILES OF TREASURE. The building will have an inventory like a werehouse where you put your treasure in like gold/silver/copper coins+ gems etc. and when the quanitity changes of these items based(will be able to only add these specific things in it not dirt/junk) the pile of gold inside the treasury/vault will expand.
This is an example of a game that I will not mention its name but it has a similar system and it looks lovely because...who doesnt love alot of treasure.