Before you decide on your next purchase of sunglasses, take the time to listen to the Itinerent Angler's podcast titled "Sunglass 101 with Costa Sunglass VP of Product Development. It was produced on Feb 4, 2013, so scroll down to the second episode in the list:
The Itinerant Angler
One thing I learned from listening to the podcast is that glass len's cause the sunglasses to be heavier, but if you tend to be hard on them, then glass is the way to go as it will resist scratches much better than the plastic. On the other hand, if the additional weight is an issue than go with plastic, but they will require a little more TLC. Costa has a propriatary approach to making polarized sunglasses which is also very interesting and that is what drives up the cost......my next pair will be the Costa 580's.