I know I have seen some used on Amazon sell for around 20 dollars as of late. That's just for the Kinect itself. I've seen them new for about 50.
Trade in value, especially at Gamestop, is not looking good in afraid. Unless things have changed I would say if it's in good shape you might get 15 bucks cash for it (that's being generous). Store credit possibly 20. With the next ggeneration systems really stating to take root I honestly would not get my hopes up to high, sadly.
I would check Best Buys TIV, or even Target etc, before I headed for Gamestop. Then again I really dislike GS so I may be somewhat biased.
I know GS doesn't like to tell you the values of stuff over the phone, but you might try calling your local on and saying "I have a Kinect in good shape etc etc" and see if you have any luck.
If you're going to be making a trip to Gamestop anyway, I would say just take it with you and see what is said in person.
On a related note I have a nephew who was friends with the people that worked at the Gamestop he frequented. Try chatting them up while there and they may offer you a little more for it. It's worth a shot if you do.
Sorry I wasn't of more specific help, maybe someone else can be, but I wish you all the best luck my fellow CAG with whatever you decide to do.
