I have the same philosophy as 33girl. Right now I'm sporting a pair of black framed glasses that cost a total of $3. I've had them since February. Before them, I had an identical pair that also cost $3 and I had them for a few years.
I had a great pair of Dolce and Gabanna knockoffs that I got in NYC and I lost them
My boyfriend lost his Oakleys less than a year after getting them. I got him some Faux-kleys in NYC and he kept them for a few years - until they broke. His parents never noticed that the $100+ sunglasses they bought him were gone