Probably not going to get too much, i'd say $200 or less around there. There will be suckers that will overpay, but if they wait, the prices will go down.
There is a flood of Lumia phones on craigslist due to the Microsoft Smoked promotion where they were giving away free phones for losing the contest. At first, it was Titan and Focus S for ATT that were being given away, as the inventory for those went quickly, they started giving out Titan 2s and some stores started giving away 900s. In my area, i saw a ton of postings for brand new 900s for $300 and started to trickle down to the $270 range.
So if that is the going rate for brand new, you shouldnt be getting more than $200 for a used one, if you do, you will be lucky.