Handheld Game System for my son?

My soon to be 9 yr old son has asked for a Nintendo DS for his birthday. I would like to get an older system that would have cool games but sell for a lot cheaper. He loves the Lego games, Pokemon and Mario Brothers. Could some of you out there help out a confused Dad and suggest a system and why it would be a good fit? If we could get a console for about 50 bucks and some games for less than 20 it would be ideal. Thank you for your help - I don't want to blow this for him.