Actually, it looks like I might not even be getting the proper income from farming:

I just started up Julii on med/med on Imp campaign to run some tests and at very high tax rates in Arretium and Ariminium they have an income of -98 and -315 respectively. Now I can't test my unmoded version without reinstalling, which I'd like to avoid, but that seems very low to me.

Current income from farming is 80 in each city with a Land Clearance.