The main concern of companies who make and sell replicas is to make a profit and sell look alike glasses. By no means at all are they putting a guarantee on the box that says they will last as long as the original pair.
Another factor for the economical price is that fake Oakley manufacturers save lots of money on research. They simply pick up original Oakleys, gather some information of the originals from the websites and produce a replica. The amount of money saved on research is so huge that the best quality replicas may come very close to the original, in appearance as well as in quality! Sunglasses are sold at discount stores and dollar shops, and the cheapest of them by street-side vendors too. It is quite obvious that they carry no guarantee or warranty cards, and they often come without any proof of purchase. The buyers are fully aware that they are picking up a knockoff Oakleys that is uncertified and unreliable.
Most buyers of "Joakleys" are students or people who lose their glasses frequently and simply cant justify the high price tag. They want to keep the costs low yet maintain the fashionable image that the Oakley style can provide.
More often than not the sales of replicas will cut into the sales of the original brands. It can be damaging if people do not know that they are getting fakes and the fakes turn out to be lemons.
It is necessary to mention that the buyers of replica sunglasses and the people who buy the original ones are from entirely different classes of the society, and there is very less possibility for one of them to 'cut into' the business of the other.
No doubt cheeps lend a temporary jubilation of purchase because of the cheap price, but the protection to the eyes from UV rays is also too less.