Any estimate on what I would get at Gamestop in comparison to eBay/Craiglist, etc?
NEVER trade to Gamestop, unless you need the cash yesterday. In almost every case, ebay will get you at least double your money, maybe even more. Go on Ebay, and look at comparible products to yours, and see what they are going for. Do you still have the box, manuals, ect. The more complete the better.
Most people trade at Gamestop because they don't have the time or the desire to deal with Ebay. Since I began using ebay back in the ps2 days, I don't think I have ever traded anything in to them. They may buy your Xbox for 100, and turn right around and sell it for 200(just an example). Now if you do the work instead, you might make that money instead.
Now sometimes they do have some good trade in offers where they give more value at a certain time and what not, so sometimes they can be decent, but that is very rare. Also, here in Vegas where I live, I beive there is only 1 store that gives cash. All the others are store credit only, and the cash is always less than the store credit would be.
So if your willing to put in the time and effort, Ebay is almost always better, but you need to see what other systems in your range are going for, as Gamestop may be better if your system isn't up to snuff. If I personally had a like new 360 with all contents, I could get probably get no less than 50 under retail.
Chris