Yeah, $500 (+ or -) seems to be the going rate on Ebay.. I would pay that for one in mint, but not for a banged up one.. They blue-book at around $200-250, having looked it up at work one day, so stores usually sell them for cheaper, probably around $300-400 (the ones who don't watch Ebay, that is)..
You can always find steals though, as I got my 750 for around $300 in '94, it was a minty fresh '88 model.. I was drooling on the counter when I saw it(the kid maybe played it three times in 6 years).. *