$11-13K I would think. Depends upon mileage too of course. If you want a convertible and that's your price range you may have to go to just south of 100,000 miles to stay in your price range.
I saw an '07 GT in the classifieds last week for $15,000, and based on what I've seen you'll be looking at quite a bit less than that if you want older than an '07. I bought mine used in '08 for $17,000.
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