That is a tough one:coffee:, but following the market the last couple years, a nice TTA that has been driven should go for no less than $15K and up to $20K. I would put your TTA closer to the $20K mark. The only problem is with these cars, there is always 5-10 for sale, simple economics of supply and demand keeps driving the price down. I think there are at least 2 on Ebay right now and do a google search and you will find about 8 more for sale.
So it is still a buyers market, so you have to decide how bad you want to sell the car and price it accordingly. I know I wouldn't sell either of mine right now for no less than $15K and both 50K mileage on them.
On a couple of positive notes, I have noticed that people who are in the market for a TTA and do their research are willing to pay a premium price for a well maintained TTA. I also spoke to a classic car appraiser at our local car show last month and he said that nice third gen Trans Ams are really hot this year and people are really starting to buy them and that will help the values go up too.
Good luck your decision, not an easy one for sure.