When I was trying to trade in my dsi towards a DSi XL (BIG SCREENS) they would only give me $90 for my DSi so I just decided to keep my DSi. They will probably take it for $70 when the 3DS is released though.
Heh, maybe $70 if you're lucky. Wasn't the GBA trade-up to DS something like $30? And DS was only $150. I'm expecting 3DS will be more, if not THAT much more (I sincerely doubt they're going to price it higher than the Wii... that would totally undermine it as a console). Because of this, I'd expect anywhere from $50 to $70 toward trade-in. This IS Gamestop we're talking about, after all. They always make profit off of their used game/system sales, but $100 for a DSi after 3DS is out will kinda be pushing it.
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