I think if you look hard enough and are patient the odd 2005 plate M3 CS does pop up in the 15-18k region which are nice cars.
It depends what you want, 13k is considered a bargain for a V8 S197 Mustang, gotta remember most people buy Mustangs for a hobby and half the enjoyment comes from turning a poor-adequate car into a great one, even more so when your doing it yourself.
Problem is that stock 15-18k M3 CS will run circles around a stock V8 S197 Mustang. If you sort the handling on the Mustang, brakes plus suspension ideally budget for 2k-3k to do so unless your going to cut corners. Then if your a good driver you will keep up with an M3 CS, but the M3 is easier to drive faster as its more forgiving closing to the limit.