Plastic and glass. The coatings can make a difference but money has virtually nothing to do with the quality.
If prescription eye glasses are marked up hundreds of dollars for a brand name, and they have individually ground glass lenses what do you think the cost of mass producing sunglasses is?
Sure there is garbage out there, as I said before, and you do have to compare. The information is there, ignore it if you will but don't suggest that I have to.