I'm due to get a new pair myself - doing a little research and I came across this:
You may wonder why there is such a significant difference in the price of polarized sunglasses and if the extra cost is worth it. Glasses can be inexpensively polarized by attaching the film to the outside of the lenses. Two factors make this method less desirable. The film will eventually peel away from the lens; it’s just not as durable. Also, precision placement of the film is crucial. If it’s placed off the vertical axis some polarizing powers will be lost. Higher quality, higher priced glasses feature lenses with the polarizing film sandwiched between lens layers, encapsulating it. Placement is aligned exactly for optimum performance. This process results in superior clarity, better peripheral vision, and lasting durability.
I have had the lenses with the film on the outside, you have to be careful not to leave them on the dashboard on a hot sunny day as it can damage the film. Next pair I will definitely be asking whether the film is on the outside or sandwiched between the lens layers. My guess is most of the premium priced glasses use the sandwich method, but I would certainly check beforehand.