I don't know about the others but I actually got mine off eBay! Most of the people who got it for that price got it second hand. They turn up from time to time so just keep checkin the forums, eBay or buy and sell. Good luck!
The 1650 price is used from either ebay or the "buy/sell" section.
If you are going to go the supercharger route, it may be beneficial to research UpRev and how they can be used to eliminate the piggyback system and to give you a proper tune. It could help you put out more power and keep it safer.