You don't buy cosplay costumes. You make them!
But it really depends on what you're going as and how thorough you want to be. I found a picture on the intarwebs of someone I went to high school with that took a bunch of cardboard boxes and spraypainted them to look like a box of pocky, and that was their costume.
Then you have people like that Brazilian chick who's semi-famous for dressing up as Samus. You know she paid the big bucks for that.