Something like this in the capabilities section of the governor buildings

Code:
taxable_income_bonus bonus 5 requires factions { all, }
nb on the building info scroll these bonuses display as 1%, 2%, 3% etc depending on the value but in fact they give a fixed amount of extra cash. The fixed amount may be constant over all city sizes or may go up in jumps but when i tested it briefly each point of taxable bonus gave 20 denari. So a bonus of 5 gives the city an extra 100.

Pretty useful bonus imo.