My recommendation is to check eBay for suits and whatnot. And a suit can essentially double as a tuxedo, you just need to buy a tux shirt and a bow tie and a vest or cummerbund basically.
But yeah, check out eBay regularly and go for designer brands or store brands, as they can go surprisingly cheap, just make sure you find out what your measurements are (go to a store and have them fit you etc).
I got a Banana Republic Suit for $35 that retails for $300 or so, and a Hugo Boss suit for $31.99, and Hugo Boss suits usually retail at $795+ (
http://store-us.hugoboss.com/HUGO-BOSS/Menswear/Suits/21100,en_US,sc.html).