How about an order of magnitude cheaper than those safety glasses?
No joke, I've been using these for a couple years and love them:
https://www.safetyglassesusa.com/pyramex-ztek-safety-glasses-with-gray-lens/
They are legit and meet safety specs:
http://www.pyramexsafety.com/products/eye-protection/S2520S
There was a thread on the slowtwitch triathlon forum where a bunch of cheap sunglasses were suggested, including the Ztek's and a couple other models on safetyglassesusa.com. I ordered several on a whim and really like those Ztek glasses. Previously I swore by the cheap Performance/Nashbar glasses. I've got both lying around and usually grab the cheapo Ztek's.
I love that they are so cheap I can always carry an extra pair with me. Having a clean pair to swap to at the top of a sweaty climb is a luxury. So is having an amber or clear pair at dusk or if a thunderstorm hits.
It's always seemed crazy to ride with expensive glasses when they are going to get sweaty or muddy, then scratched up trying to clean them with my bottle and jersey.