People are selling them for about $400-$450. Seen them as high as $500 but most of those where brand new in box. I am certain you will be able to get $450 since the phone is back ordered till about late this month.
I paid $550 for a brand new one never activated, with a fwe extras on ebay. As for $400 dollar Droid X I have not seen one....look at completed listings on ebay 90% are over $500. I looked for a while and saw very very few go for less than $500 and most for $520-$570. Also there are very few on Craigslist. You should easly get $500 for it if in like new condition.