Not all wheels are created equal. Porsche's wheels are forged, which is a process that creates a very strong, light wheel. Most of the cheap-os are cast, which uses a softer alloy of aluminum and without the work hardening of the forging process, they are much easier to bend and damage. I'm not saying the lower price isn't justifiable, but you are getting a better product by paying more (in general).
Here's a good read:
http://www.sportcompactcarweb.com/tech/0205scc_wheel_tech_guide/index.html