Morethanmary.com's Mary Rambin, a self-described "sunglasses brand whore" shows off her sunnies steal: oversized Anthropologies. For just $4 (with UV protection)! Nice find, MTM.
Anthropologie's websites doesn't have any deals of the under $10 caliber currently, but there are plenty of finds for under $50.
If I was buying from their site today, I'd go for the Gumball sunglasses in blue. For only $24, why not opt for some color? At that price, when you get sick of them in a month, you can snag yourself a new pair.