Just to add a data point, I'm 42 with atrocious eyesight (~20/2000 in one eye, 20/600 in the other -- I wish I could remember the diopter values, more accurate!) and my glasses usually run about $425/pair. I have one spare set. Can't have the surgery, my corneas are too thin to chance it. So I'd jump at a chance for cheaper spares -- I'd really be blind without them.

Must set a mental note to write down my glasses prescription and store it somewhere accessible...