You know where a good place to ask would be? Gamestop. You know this board is for Wii owners not former Wii owners; wouldn't imagine why someone who has sold a Wii to gamestop would be on here. So asking here isn't very likely to net you an accurate answer, call gamestop in the morning. They may even have the trade in value on the site.
you want to sell your wii for a dsi? even though wii motion plus just came out and games like the conduit, re, dead space, wii sport resorts, mario, and many more coming out this year? wow, um...you'll get around 150. do i know for sure? nope, just guessing.
Why in the world would you trade your Wii in? Motion+ just cameout and Tiger Woods 10 and EA's Grand Slam Tennis use it beautifully. Then you have The Conduit, Wii Sports Resort, Super Mario Wii, Red Steel 2, Super Mario Galaxy 2 and much, much more coming in the next year. The DSi can be saved up for.
As far as trade in value at Gamestop..I'd be surprised if you got more than $100 for it.