It's perfectly reasonable that prescriptive sunglasses are more expensive than regular sunglasses. There's far less demand for them, so the price has to increase to make up for it. Is it fair? Yeah, it is. Let's turn this around. Is it fair that glasses are more expensive than store-brand reading glasses? These lenses aren't cheap because they have to be nearly personalized to the individual's eyesight. That's all there is to it. It's a business. They're not there necessarily to help peoples' vision. They're there to make money. It's as simple as that.
I went to the opticians on Saturday to find that my new sunglasses would cost me around about £150! Because I am a child my eye sight keeps changing and whenever I go on holiday it changes and I need sunglasses so image £150 paid a year. So not fair!