Assuming it is the 2.26 GHz C2D, probably around $550-600 or so.
Look on Craigslist to see what machines like yours are listed for, and look at eBay's completed listings.
When you list it, make sure you provide a good description and a couple of good pictures. Say whether you have the box, restore discs, etc.
Also know that there are usually a number of the i5 13" MBPs floating around, and they're roughly twice as fast (on benchmarks, at least), and will still be under some sort of factory warranty. That has really hurt the resale value of the Core 2 Duo MBPs (and not just the 13" models).
Mac2Sell and Amazon are OK starting points, but it's like the blue book with cars - the blue book can't hand you cash or write you a check. And the prices on both are not actual selling prices - and on Amazon in particular, some of the used prices are nothing short of outrageous.