happens all the time. look at iphones they also lost value not as bad as nokia but from what I've seen now 4s going rate is around $350-400 for 16gb model. Unlocked ones go for more. Happens all the time. Also one thing I noticed from iphones is that its better to keep the phone a bit longer and let the dust settle down when new phone comes out. Usually first week upgraders are lucky to snag new iphone and keep driving prices of previous version down in order to dump them fast. After a while it actually levels off and with release of unlock and jailbreak value goes up a bit.
Once ATT releases unlock codes for lumia 900s they will go up a price a bit as that opens up a different market. But overall all phones drop in price as time passes. early adapters move on to the next phones etc.