Unlike other products, glasses are custom made products. Glasses are professional vision correction tools for people with vision problems.That is why they are costly. But you can still find some online stores offer glasses with high quality but with affordable price, usually 1/2 lower than that in brick and motor stores. Firmoo is one of them, they have large selections of frames,but you can save up to 80%-90% for a pair, my last pair cost me only $20 with shipping.
Prescription lenses need to be customed, which need quality advice and professionals to complete them.So you need to pay an expert to fit, dispense and adjust the prscription glasses.Usually a quality pair of prescription glasses does cost much.
The main reason that prescription glasses from your eye doctor cost so much is because one company Luxottica own the majority of the companies that make the frames and so with a virtual monopoly they can charge what they want and get it.