They ARE overpriced, but they are NOT cheaply made. They are excellent eyewear.
I obviously don't know what you "found at the beach." Oakleys seem like they are kind of flimsy, but the plastic is virtually unbreakable. Okay, yes, if you actually WANT to break them you can snap them in two, but it takes a surprising amount of force; much more force than you would ever experience in normal wear and tear. Most of their lenses are very high quality as well, although they do scratch rather easily. Replacement lenses are available for every model.
I have a pair of half jackets that I bought for cycling and other sports 4 or 5 years ago. The never slip, and I hardly ever even notice they are even on. I did have to buy some replacements for my G30 lenses after about 2 years of use, but the other lenses are all still in great shape.