depends which country we are talking here.
In the US, assuming very good condition for a 10+ yo car and some bells and whistles attached (premium wheels, leather, spoiler, premium sound, etc), I'd say $6000, give or take, would be a fair "starting" price. Would only consider buying such a car, however, if are talking from a sole, adult owner. You have a far better shot at getting a well-maintained, well-cared for car that way.
You don't wanna be buying high-mileage sports car from a guy who's the 3rd owner and just started shaving 2 weeks ago, so to speak.