I paid $500 got my D&G and they are fantastic. Though I did drop them and have a micro tiny mark on the one lense that looks like a dust fleck. My Prada's were $450 and they have taken a beating. I wear them in my helmet when I go riding, so they are on, off, on, off a lot, and they are still perfect.
However, the best part about cheap sun glasses is that if you break them or lose them, you can but 40 more pair before it reaches the cost of a really expensive pair. Ya don't really care if you break or lose a cheap pair.