Just as one shouldn't skimp on protecting one's hearing, they also shouldn't cheap out in protecting their eyes from damaging UV rays, especially when operating at high altitudes for prolonged periods of time.
I use to wear prescription sunglasses that were very small frame and lens wise. After an extended period of work (anything > 6 hours) I had significant eye strain. Last fall (2008) I transition to contacts and purchased some full sized Randolph-Engineering sunglasses, and was absolutely amazed at how over the same period of time or longer while flying I actually felt great. No more eye strain.
I tried some $15 from a convience store, and the plastic lenes were absolutely horrible - but I needed something while I waited for the Randolph's to come in.
Glass lenses > Plastic.
Quality > Cheap.