I once bought the aviator sunglasses at duty free shops at airport with cheaper price than that at Market. Because of the save of duty, the cost of sunglasses will absolutely lower. However, in order not to buy the false one with big brand, you'd better know the skill of judging the truth from the false.
Though i didn't buy sunglasses from duty free shop at airport, i bought other products from duty free shops at airports. They are cheaper than they were in supermarket. So, i believe sunglasses cheaper at duty free shops at airport. You can have a try. Or you can also search it at online shops where usually average cheaper than they were in real shop. Good luck!
At the duty free shops, you can find many cheaper goods, and the prices of these goods are lower than the prices of goods in marketplace. So I think you can find cheaper sunglasses at duty free shops at airport. But not all of the shops have the goods which you want to buy, because the goods at duty free shops at airports are limited, and not all of the duty free shops at airport are same, meanwhile, at some place, the price of goods at duty free shops at airport are not much cheaper than the marketplace, hope you can find cheaper and suitable sunglasses at there.