alot of the games are cheeper too its just like the Wii games usually go in price by quality, and how long its been out on the market. I have bought new ds games from 10-40 dollars all depending on the game, same for Wii usually 20-50 bucks new.
Although i would say dont buy it just for the DS demos off the Wii. that would be pointless and i doubt it will be that great to kepe you satisfied for the investment. There are tons of other really great games you can play on it, aswell as some cool GBA games. The reason the DS is so cool is because its just as inovative of a handheld as the Wii is a console... If you've owned a handheld before your going to have to get use to the touchscreen play with some of the games, but its a blast and there is alot you can do on it... If your willing to buy a few games to go with it its a great investment and will keep you busy.