[FierceGT]

Personally, if it is a 4 cylinder, I wouldn't even consider it. If it is a 6 cylinder, it runs, and it's in decent shape, it would probably be worth around 800-1200 bucks. In great shape with that many miles, maybe around 1500. The GT's of 85 are worth more than the others of the same year. And if it's a 6 cylinder, it probably doesn't really need a new muffler, they are naturally loud to begin with. Check around it for any leaks to be sure.
If it's a GT in good shape, 500 is an excellent find.