I haven't bought prescription sunglasses for years and decided it was time. After receiving quotes of over $100, I found a clip-on that fit my prescription glasses (I updated these recently). These work best with metal framed glasses and were just a few dollars at WalMart.
I also came across a web site for reduced price prescription glasses at 39 Dollar Glasses. I haven't purchased anything from this site, but it looks like it's worthwhile to take a look. Currently, there's a coupon on Frugality is Free