Hmmm, I think Sennheiser technically discontinued the HD600 and 650, so that may be why. That's a bummer :(. Scouring our FS forums here is a good idea as well. I'm not sure what they go for, but getting one that probably looks as good as new is probably very obtainable for $400 or less.
Sennheiser in the UK is still listing the HD600 and the HD650. Not at US prices, though!