As I understand it, the FI rating basically goes through the PPL again, but this time you're in the right hand seat. The instructor will show you how to give each lesson and you then 'teach' that lesson back to the instructor. However, if you want to get paid for your instructing, you will need a CPL too. I think the FI rating is a similar price to the PPL, but will depend on where you train.
I don't speak from experience, so please feel free to correct me if I'm wrong.