Not enough to make the trade-in really WORTH it, if you ask me. As long as it works 100% and isn't on it's last legs, you can try trading/selling it here for like $50 or so. But, that's only if it's complete(has all the wires and at least 1-3 controllers) and has never been opened, since the only reason to really open an Xbox aside from modding it is to clean the laser if it's failing.
I'm actually thinking about doing this myself with my 2005 model Xbox, since I haven't used it in over 2 months I'm sure. But I'm still undecided on that, since I know I'd probably want the shipping costs on top of the trade/sell price because it costs about $20 or more to ship one.