My wife got a pair of sunglasses from Zenni Optical http://www.zennioptical.com, and with a bunch of add-ons like scratch guarding and even with shipping it came to less than $30. The worked well, but the frame broke at the hinge after a few months. Kids ripping them off didn't help, but still, you get what you pay for.
I'll probably get some soon since I trash my lenses pretty quick. They seem to be incapable of making a safety lens that is also hard to scratch, and there isn't much that's more abrasive than metal dust.