Of course, I have heard of them. They are very famous brand which are used by many people who like fashion. But on the other hand, they are kind of expensive. For the prices, because they have different kinds of types, the prices are different too. But it can cost you at least 100 dollars. And the high price can be more than 2000 dollars. So we can say they are the luxury goods for some people. And I will suggest you buy them on online shop where they are cheaper than the real ones, and they are also of good quality. So you can just link to http://www.sunglassesuk.com/brands/tag-heuer-sunglasses/. Besides, when you buy them online, you should pay attention to the warranty so that you can send them back, if you do not like them.
I just search online and find there are several sites provide tag heuer sunglasses. And there prices are very different from each pairs, you can get a pair at less than $300. But the hightest tag heuer sunglasses need over $1000. So, how much should you prepare depend on which type of tag heuer sunglasses you choose. Here is a sites provide tag heuer sunglasses: http://www.framesdirect.com/framesfc/Tag_Heuer-lamhra-slb-l.html
The average price of tag heuer sunglasses online is about the $ 130. You could buy them from the online stores which will absolutely cheaper than those in the franchise stores. You could go to the amazon to have a search because there are a lot of sunglasses. Maybe you could find your favorite one there.