This is really interesting. I love such economic aspects of the game. Here's some ideas.
-Corruption could drasticaly decrease income.
-Mines could be engineered to dry out after some decades.Especially if the player chooses to upgrade them.
-High fertility rates could be perhaps linked to the rise in the cost of training and building??? (remember more people>food prices up>everything becomes more expensive).
-farm improvements and long range sea/land trade (ports/roads) diminish inflation (thus reducing costs).

i dunno if such stuff are hardcoded or can be changed by the EB team.