GameStop used to sell refurbished ones for $99. I bought one last Christmas and it is still working. Although maybe it was just a DS, not a DSi??? Might be worth looking into. If I recall, they had some type of warranty on it.
If you have any games or systems you'd like to trade in, take them to gamestop or EB games and you can get a store credit to put towards your purchase. They also have great deals. Good luck!
Target sells the DS Lite for $99. We have got a DS Lite and DSi...my almost 4 year old plays the DS because it can play Gameboy Advance games. The big kids play the DSi. The 3DS just had a price drop from 249.99 to 169.99. They play regular and 3D games.
We thought ours was lost so I checked with our local gamestop and they were $50 used. Ebay also has very good deals, especially if you find a package with games that you want.