What it costs is really depending on what you want from it. If you just want to play and have fun, it doesn't have to cost much. Recently I discovered standard tribal: that's standard (type 2, the latest sets, for the n00bs
)with the requirement that 1/3 of your deck consists of creatures of the same type. Those formats with restraints are nice, because it keeps you away from the most expensive decks. And playing with a handicap makes it ok if you loose. And there are also tournaments with tribal standard.
Such a deck won't cost you more than something like $20. The amount of time you spend on it, well, that's up to you. A single game doesn't take much time, a league will take at least 3 hours a week. A league will cost you about $20 as well (too much if you ask me).
The bad thing is that once you start, you always want more and better cards.