The first thing you should know about sunglasses is that a massive percent of the worlds sunglass lenses are made by one company, Italian manufacturer Luxottica who make lenses for brands like Ralph Lauren, Oakley, Mai Mai and a great many other high profile fashion brands. They produce only 4 or 5 levels of "quality" the top few being polarized. Therefor you may pay $40.00 for lenses of the same quality as a $400.00 pair or choose to spend around $70.00 to get some great frames aswell. The only thing you won't get for this money is a shiny (pronounced wanky) designer lable.
Personally I tend to bugger 'em, loose 'em or have 'em stolen by some arse sooner or later so I never spend more than $30-40 each and when I find a pair I really like I tend to get 2 or 3 pairs and keep the spares in my dresser for when the inevitable occurs.
That's my 2 cents anyway.