I'd recommend to you:
Yu-Gi-Oh: Many people say that it's expensive, but actually it's not. Most pf the best cards (excluding just a couple of Power-Rares like Mirror Force) have been reprinted many times as Commons, so just buy a starter or structure Deck and you get about 5 to 9 of the best cards ever released.
MtG: It's pretty cheap to get many cards really cheaply , especially from me

Pokemon: I mean, nearly everybody has got a box of some old Pokemon cards and they'll be selling it to you for just some $, the game itself is actually pretty awesome.