Price is not the best factor to value how well sunglass lenses block UV rays. Some expensive, designer sunglasses don't provide good protection, while many cheap sunglasses do. When you are choosing sunglasses, make sure the label says that the lenses block 100 percent of UVA and UVB rays.
No. That whether sunglasses have 100% UV protection has nothing to do with the cost. Some cheap sunglasses which are labelled with 100% UV protection can also give you 100% protection from UV rays. So when you choose sunglasses, just check the label to see whether they have 100% UV protection.