I watch shopgoodwill.com fairly regularly. The black ones go for the $250.00 range, a little more for a nice one with accessories, manual and a nice case. They go for less if the don't have any of those things. Add $150.00 to $200 for white. (The prices seem to be down a little right now, though.)
They area asking $100.00 to $150.00 more here on DFW Craigslist, but they sit a long time. Whether they actually get the asking price is a good question.
bk