Here's a link to "The Unofficial Used Tandem Price Estimating Tool":
http://www.thetandemlink.com/usedhome.html
If you work through the process on that page you can come out with a fair number. There's also lots of good thought-provoking info in the text that accompanies the tool.
Also, FYI, the web page and the Tool are created by TandemGeek, who posts here on this forum.