The price of sunglasses is vastly different, in addition to the brand to determine the price, as well as lens materials. Lenses can affect optical clarity, weight, durability and cost.
It can block 100% UV rays by itself and does not require lens coating. Excellent impact resistance and transparency, second only to polyurethane. It's cheap, light, and not resistant to friction, so it needs a film.
Maximum impact resistance and optimum optical transparency. Flexible, light and expensive.