I think trade-in and used prices for iPads have dropped substantially in the past few days, as most people have made their Christmas purchases. The money that people got for their old iPads just a month ago is no longer there. I traded in my iPad 2 32GB wifi to Amazon for $345 in mid-November - now the maximum they'll pay is $208.
This is not a good time to sell or trade - it might be worth holding onto it for a month or so if you can, and you might get another $50 - $100 for it. On the other hand, I think the cellular iPads don't hold their value quite so well as the wifi versions (smaller market), so 50% of the value seems pretty reasonable to me.