If you insist on gamestop, the only way to not get screwed is to stack trade-in multipliers. Your edge card combined with tier trade in offers(ie 2 games for and extra 10%, 4 for 20%, etc) is the only way to trade at gamestop. Otherwise, they are just stealing from you.
But to answer your question, maybe $20 if you are lucky.
The way to make GameStop worth it is watch for when they have big trade-in bonuses like an extra $10 every 3 or $20 every 4. Makes a great value if you have a couple cheapo games to trade in. Otherwise ya probably just don't use GS.
Sold my Wii, got an Xbox 360 :D Xbox Gamertag: Zepaw