I finally bought some new sunglasses to replace the ones I lost in Harvey Norman. I learnt the difference between cheap sunglasses and OMG expensive sunglasses, too - admittedly from a biased saleswoman, but I found it interesting nonetheless.
The cheapest lenses you can get are acrylic. They have the worst optics, and if they're polarised the film will sometimes just be on the inside of the lens. Although it may not be apparent from trying them for a few seconds, you can get a good idea of the distortion by holding them up facing you and looking at your reflected face. Move them around and see how the face distorts as it moves across the lens.
I looked up lens types on the web... it seems that acrylic lenses are discounted in all comparisons. Comparisons appear to be between polycarbonate, CR-39, and glass lenses only. In order of price:
Polycarbonate lenses are a big step up from acrylic. They claim to be "distortion free", but there must be some room for improvement to justify the more expensive lenses. Polycarbonate lenses have the advantage that they are the lightest and most resistant to shattering. However, they are the most prone to scratching, and have worse optics than CR-39 or glass.
CR-39 lenses are right in the middle. Optics are better than polycarbonate but worse than glass. They're heavier and more likely to shatter than polycarbonate, but lighter and more resistant to shattering than glass.
And finally, glass lenses are the most optically correct, the heaviest, the most likely to shatter, and the most resistant to scratching. It's also the most expensive :|
The glasses that I lost were polycarbonate Zenith sunglasses. The ones I bought look almost the same - they're polarised CR-39 Zenith. An excellent replacement!