I have a pair of sunglasses that I bought for $7 on amazon. I've been seeing a bunch of people with the same sunglasses in various colors. They're sold through a bunch of different sellers, some factory is obviously pumping them out.
They're surprisingly sturdy. I originally bought them only for driving and was surprised at how well they stay in place while running. YMMV depending on your face shape, of course.
Here they are from one of the sellers, they're also available from other sellers, this was just the first I found right now:
https://www.amazon.com/Retro-Rewind-Classic-Polarized-Sunglasses/dp/B00NWJ1OV0/ref=sr_1_1?ie=UTF8&qid=1497377082&sr=8-1&keywords=sunglasses
They're polarized too, which is nice when I run near water (SF bay and alpine lakes) to cut glare.
I've had expensive sunglasses in the past, but I always get upset when I lose them a year or two later. I only buy cheapos now. For the most part, if you choose cheap sunglasses wisely, you're getting the same level of quality, unless you want nice titanium frames or something else only available from a high end manufacturer.