That is also why you don't always exterminate when you take over a town. More people means more money coming in. Maybe a bigger cash payment right off, but it takes longer for it to be effectively prosperous. I usually check the public order before deciding what to do, looking at the color of the unrest. If it is city level, especially large or huge, red means exterminate. Also, after a revolt or conquering an Egyptian city, I exterminate, but other than that, usually not.