We pay for prescription sunglasses, not only for the materials to make sunglasses, but also the whole package of the sunglasses including the design, publicity, and prescription.
The manufactures,retailers,and doctors need to make profits over the sunglasses.
The cheapest place to buy prescription may be the official web of the manufactures. Some of the manufactures have their own websites to sell their own glasses, they save a lot of procedures, so you can save plenty of money too.
By the way, they are professional because they also have their own doctors.
Compared with the normal sunglasses, the prescription sunglasses will absolutely expensive because of the additional functional at the lenses which will not only make you see clearly but also protect your eyes from the uv rays. You could still find the relatively cheap prescription sunglasses from the online stores, like amazon, walmart and so on. Their price must be cheaper than those in the real stores.