Trying to learn about rear sets?

So I'm just trying to get some info about rear sets. First, why are they so expensive and what kind of difference to you get between a 100$ vs 500$ or 800$ set?Second, what kind of difference to you get over stock foot pegs? I know you can change the position, but don't know much elseAny other advice for a novice? Thanks via /r/Trackdays https://www.reddit.com/r/Trackdays/comments/919lg2/trying_to_learn_about_rear_sets/?utm_source=ifttt