Older mustangs are nice, got to love the Cobra's and I am sure you could find a nice one with low miles for around that if not cheaper. Still is a gas guzzler though.
If you want something quick and is decent on gas then check into the ST Focus like I have. You could find some around 12k and if you were to dump another couple g's in the eng, then look out. I have put about 1300 in mine and doing low 14's with it, but is my DD and wont do much more too it. Can hang with quite a few cars and is the reason I am selling my Eclispe GTS, LOL!
If you want a car to hold resale value if you are looking to sell in future then a nice Honda Civic might do the trick, scion or an XB will save you on gas even know those are fugly looking.
Shit there are a shit ton of cars that will do good on gas and are quick for around 15000 smacks!