I can't really tell you one way or another. There's a member on here that bought a '98 GT from a dealership about a year ago and paid over $9k for it, because it was exactly what he wanted. What I'm getting at is the car is worth whatever you're willing to pay for it.
If you have a budget of say $12,000 and this car provides everything you're looking for in a vehicle (engine, body style, trans type, color, interior options, etc...), I say buy it. OTOH, if you'd prefer a newer Mustang, a Cobra, or a Mach 1 perhaps you want to keep looking.
My suggestion is to do some research on similar vehicles in the area (sales price, etc..) and if the car seems in the ballpark for the sales price go to the dealership, take it for a test drive, and decide whether or not to buy it. If you find other GT's in the area selling for much less, then go to the dealership, test drive it to decide if you want it, and haggle with the dealership by showing them similar cars selling for less in the area. Hope that helps