The problem is you have to get the distance between your pup's measured for the lenses to be made right to for the frame (or at least that's how I understand it), and the measurement is hard to do on your own and they don't give it to you with your prescription.
In my case, I have a strong prescription which requires high index lenses or else the lenses come out ultra thick so the price goes really high even on the online sites, and that's IF they sell high enough index lenses.
My last pair of glasses cost $500, but it was worth it. That said, I just got a years supply of contacts for$200 and the prescription quality is really great, much better than my previous experience with contacts. I have stigmatism so contacts don't really work well for me, but the freedom is great.