If you are looking to buy, you can find them real cheap from time to time. There is usually someone out there with a starting bid of .99 cents and only one bid, so someone got a real bargain. On the other hand, if you price it right you can get more than $20.00, especially if you have a store on ebay and can leave it for a few weeks or months. Eventually, someone will come by and buy it. Someone who wants it right then and there. Also your reputation is a factor too.
I have over 17 ti99's now with 2 expansion boxes and boxes of carts, spare parts, etc. I'll end up putting them in store and should have no problem selling them for around $60.00 or more, even in these trying economic times.