I brought one pair of prescription Oakley glasses (not sunglasses) online for about £180 then my current pair from a local recommended optician for nearer £300. I have to say the difference in lens quality is massive. I have a prescription which means I have to go “double thin”, in the cheaper ones the lenses stick out very slightly from the frame, and you can see they haven’t be seated with as much care, the new ones are much more neatly seated in the frame and are thinner (even though they were sold as the same type). Also the scratch resistant coating is actually scratch resistant on the new ones and if I do manage to scratch them they’ll replace them for free.
I guess its just a case of how anal you are about attention to detail, the cheaper pair were a perfectly functional pair of glasses, but the more expensive ones are just that bit nicer.