If you don't want to pay much for the brand, you should leave out the designer sunglasses such as Rayban, Prada, Gucci etc. Some copied styles of the high end designer glasses also come in good quality and low price. It is a good choice.
I can share some of my experience with you. If you want to find sunglasses that worth the best value, it is a good choice to get some discounted designer sunglasses such as Rayban. There are some online stores that specially provide discounted designer sunglasses. You can search "discounted designer sunglasses" on google.