a loose one on ebay sold for 10GBP. that sounds about right, for a loose, played with piece.
http://www.ebay.com/itm/Thomas-Friends-Silver-Thomas-Take-Along-TakeNPlay-/152236840444?hash=item237205d5fc%3Ag%3AACYAAOSwFe5X1Cu0&nma=true&si=E6Zg8hX9N3s473Vx0gtnlO4uxQ4%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557
the one that is listed on amazon for $500, has likely been there for years. same with the one on ebay for a similar price. just because someone wants that, doesnt mean its worth that in the slightest.
edit: maybe hold off selling it. i saw on facebook that apprently the take n play range is being discontinued as of next year, so the prices may go up a little? not sure why its ending, as most retailers i know of carry that, trackmaster and minis. very few carry wooden on the shelf.