I used to loose sunglasses all the time until I spent the money for a pair of Oakley Flak Jackets. Cost around $150 with polarized lenses 6-7 years ago. I still loose ther cheap ones but have managed to keep these. The thng I like about them is the frames have a lifetime warranty and everything else can be rebuilt. I just spent $70 for another set of polorized lenses.
My wife has a set of Costa Del Mars. They are good too.
There are some good glasses for under $50, often for under $30, but you never know for sure until you have had them for a while. I've picked up some over the years that really were just as good as the Oakleys, but I've had others that distorted my vision or did not hold up long enough to be worth even the $30 or so they cost.