You have to know, the price of any commodities is not only based on the raw materials, but also related to the publicity costs, the brand reputation, channel of distribution, staffs' compensations and every cents the manufactures spend.
As the consumers, we are the ones who pay for these, and pay the extra money as the profit for manufactures and sellers.
The quality of the glasses is partly depends on the price.
So you need to choose it carefully, most of the sunglasses produced by celebrated brands are expensive for their fame not for the real quality.
Ok, you are wondering in what ways are those pricey sunglasses different from normal kinds of sunglasses. Anyway, as to the question , I can say, the differences lie in a lot of factors, such as the materials, the brand value, the design,etc. Indeed some designer sunglasses are much more durable than other types for their materials are much more strong and scientific.