Personally, i prefer to buy knock off sunglasses if the sunglasses look good on me and can shield my eyes. However, if the knock off sunglasses are in poor quality and very ugly, i prefer spend more money on authentic sunglasses, or give up of buying sunglasses. Since i have less amount of money, i prefer some really cheap sunglasses with good quality. But i never buy products at cheap prices but with very poor vision.
For me, I would not pay more to get authentic sunglasses, especially the designer sunglasses because they are so expensive that I can't afford to buy them. I would buy sunglasses that have similar style to the authentic sunglasses and they are of good quality, too. Because they are much cheaper than the designer sunglasses. There are so many online optical stores offering sunglasses with good quality and affodable prices.