Also never overlook the benefits of conquering and pillaging a province, and ransoming any prisoners if you find your military budget it getting too large...
During my recent single player game as the Byzantines I pretty much bankrupted the Empire to create a decent military to hold off the Mongols when they arrived, my finances went into the red as a result of my massive troop & naval upkeep.
After disbanding a few corrupt officials and sending my armies storming through Hungary and Spain, pretty soon I was back on track through the Pillaging and ransoming.
War can be good for business.