I wear my sunglasses a lot - I'm from socal and so it's usually sunny. I pretty much wear them anytime I'm outside and often end up wearing them almost the whole day (on top of my head if I'm inside). Anyways, because I wear them so much, yes, the expensive polarized sunglasses are worth it.
I spend a ton of time around the water (pool or ocean) and so the polarized lenses help a lot with the glare. Plus the expensive sunglasses will have better lenses - when I used to wear cheaper sunglasses I could really tell when I had them on. With the better lenses, the clarity is a lot better and your eyes won't get tired after staring through cheaper lenses all day.
So, if you wear sunglasses a lot, then yes I'd say to buy the more expensive ones. If you are around the water at all (or other places where there is glare) then yes I'd say to buy the polarized lenses. But if you lose/break them quickly or don't wear sunglasses that often or for that long, then you'd probably be okay with the decent $30-40 pairs.
Also, once you know what pair you want (and you've tried them on - this is important!), look around online, especially e-bay. The sunglasses I currently wear (see below) are normally 125 or so, but I got them on ebay for a total of $80 (including tax ans s&h).
