Pauper is a very cheap way to play Magic, 60 card decks with commons only. Some of the top competitive decks only run about $30, and you can build fun decks up with 10 cent cards from your LGS.
For casual play, my favourite is still Commander though. You can definitely build commander decks on a budget (one of the ones I made run me $26) and the format is a lot of fun!