Acid Rain World, someone please explain!

I have to ask those Acid Rain collectors out there. I am looking at some of these figures online. Why are they so expensive?!?! They are very cool looking, however, $20 - $30 seems to be a ton of money for a single 3.75in figure. Are they all 3.75in, or are their 6in versions? I am not judging, just curious about what makes these so appealing considering how expensive they are. Also, are they worth the price quality wise? Once again, judgment free, just trying to understand.