I don't know how this is going to be priced, but I've heard a few people kicking around the $6k figure - which is batshit crazy. You can get more performance (and a warranty!) for less money - it's called a 135i.
And no offense to the OP, but who is he exactly? Do you really want to put a supercharger onto your Al-Mg engine that someone R&D'ed out of their garage?