PMI has 3 levels of membership (May 2010 numbers):
- Individual Member: $129 first year, $119 renewal fee. This is for any active project manager who wants to become a PMI member
- Student Member: $40 first year, $30 renewal fee. This is for someone who wants to become a project manager, but is currently not.
- Retiree Member: $60/year. This is for someone who was a project manager, but is no longer an active one.
All members will benefit from certification discounts as well as courses/lectures discounts, and will be automatically subscribed to several PMI publications.
Your last question about whether the PMI membership is worth it or not is highly subjective, personally I think it is not, as nearly all the advantages, with the exception of discounts on certifications and courses, are available for free from alternative sources. On the other hand, it doesn't cost that much for a Project Manager...