My Dad has some, they are good quality and he has had them years. They won't fall apart like cheap ones but even expensive glasses a still delicate objects, if you step on them they will break, if you drop them they will scratch. The reason expensive sunglasses last longer is because people look after them more.
If you are just looking at it like an investment then it is not worth it. The reason people buy expensive sunglasses (and clothes, watches, jewllery, handbags etc) is not because they last longer, that in itself doesn't justify spending so much but because they appreciate the quality in itself. When you handle nice sunglasses and put them on you can really tell the difference in quality and even though they might not protect your eyes much more, its nice to know you have good sunglasses and not crappy ones on your face.