I'd suggest you expect between $350 and $400 (U.S.$) and that depends on the condition of the machine and what else is included. Do a search of "MC239LL/A" on ebay and then click on "sold" auctions.
If you have the original box, include a mouse, keyboard, and have up-to-date software, you'll be in the higher end and possibly a bit over $400.
That's the beauty of auctions - every time a buyer gets a bargain, a seller is disappointed. Each sale can vary.