It doesn't cost anything to rush at my school. And nobody goes out and buys a new wardrobe. The first two nights of rush are casual (jeans or shorts), and it's REALLY ok
The second two nights is no jeans, but not really dressy stuff. Sort of like business casual. The last night is really nice stuff, but not a cocktail dress or something like that. Just dressed up
If I had to buy a new wardrobe for rush, I don't think I'd do it!