Your glasses sit on your face, are an integral part of your look, and need to last for a long time while being as clear as possible. People don't want $30 glasses because they're a piece of junk and will break on you when you most need them. You can make mass market glasses for ~$100, but I think most people want a more specialized look to their face. A few hundred dollars just happens to be the sweet spot where people get to buy a product which is purchased infrequently in a store, get a high quality item, and get good customer service/support. It's sort of like an iPhone. People want to pay a premium for a device they use every day for years which will be fast and just work. Apple doesn't have a monopoly, they just provide a better service