Prescription sunglasses get expensive real quick.
I have some prescription Ray Ban's that I got from the place in the mall, LensCrafters I think? They often have great deals. The chains usually make poor quality glasses, but mine came out great. Also, Sunglass Hut has a deal with the place I went to (just ask sunglass hut who they partner with to find out who it was if you want), so you can buy the frames at Sunglass Hut and have them made at the joint and still get the great deals.
Retail came out to a little under $600 for my glasses, but I got something like half off the frames and then a free lens upgrade, so I ended up paying about $350 for some very nice RayBan frames, polarized and polycarbonate prescription lens..