As a MTG player for 8-years now, I've seen this question fly by many, many times. Magic has changed over the years since I was a kid playing. It became less of a "help me, I'm a beginner" type of game into "if you cant learn it, just leave it" type of game. Or if you cant afford it, then also leave it.
The thing about your question is that it's hard to give you one simple answer. So many sets (I think 4 sets come out a year now?) come out every year, meaning new cards, and new deck types to play. Most decks competitively nowadays are basically the same, much like how Pokemon battling is in OU -- you see a lot of the same stuff. Though, it is true, as the person above has stated that a decent competitive deck will cost you around $300. Though, the price can vary as well, and it can get near to $500 or it can be around $150 - $200.
The thing about Magic today is that with so many new sets coming out a year, it makes it harder for a deck to last. You cant just upgrade it here and there, you have to make a new one with every block. The price varies mostly on a deck with the new sets, the condition of the card(s), the block set, and the type of deck you're going to build. You can always consider buiding something that suits you as well, and don't have to play aggro or control or aggro-control or whatever is in for deck types of that block set. I mainly play combo/control, and for me it can either sky-rocket in price due to me needing some rare shit that has a high price tag on it, or it can be super cheap and a very effective deck without it being gimmicky.
There is also problems on where and how you buy your cards. Most players, like myself, go to industrial areas in my state (California) where they have sections that sell MTG cards along with like sports cards and Yugioh cards, etc., etc.. If you have a place like that, or if you're not sure, try to find one -- cards will be cheaper there as they're venders and you can bid with them on prices. Buying cards out of booster packs is pretty lame. It's too expensive IMO and just not worth it. If you're going to build a deck, then best you just go ahead and straight-out buy the cards you need right then and there. If you're rich like most kids I see playing MTG, then buying a few boxes full of booster packs also works on getting some stuff you may need in mint condition and if you don't need the rest, you sell 'em and get what you need (or save it for your collection or whatnot).
Basically, Magic isn't a cheap card game anymore. Cards are better (in some sets) and can get pricey.
~ Aether Nexus