Hang on!! Wait!! Does he have the Game Stop card?? He can get an extra 10 percent on trades and then they're often running promotions where he can get an extra 10, 20, 30 or more percent on top of the 10 percent of the trade in value. This can *really* add up! DH does this all the time, and ends up paying around 10 bucks for a new game. Sometimes the extra percentage is offered if you put the trade in value toward a pre-order but almost all the time he can put it toward something other than what he pre-ordered.
If he doesn't have the game stop card (it's like a rewards card) you can buy it for $15, but you get a year subscription to Game Informer magazine and it gets you coupons for things like buy 2 get 1 free, and the etra 10% on trades, etc. Not a bad deal.
OH!! I just saw this. Game Stop has an extra 50% (yes, 50%) trade in value when you put it toward used games and consoles. He might be able to get a used DSi in great condition for next to nothing.