I'm all for the $5 sunglasses.
I have huge, fantastic shades that I got from Walmart.. they work and that's what matters.
My first pair were a $10 one from Nordstrom (you gotta know where to look in the store), but my head is so big that they broke in a few months from taking them on and off.
The 2nd pair were from Nordstroms again ($10!) and were a little small. Not enough for me to care, but again, my giant head broke them.
My current pair are the $5 ones from Walmart. I grabbed them before I went to Europe. They are AWESOME! They actually fit on my giant head, and they cover my eyes, and block out the sun.
If I can get the same amount of protection with a $5 pair as a $500 pair, I think I'll go for the $5 ones.
(Yeah, I also buy the knockoff shirts. I will not pay $72 for a Lacoste polo, if I can get the same one (basically) for $10 on Harwin!)