The Rotrex unit is $1900

http://www.w2wpowertrain.com/pc-28-64-c30-94-head-unit-250-400-hp.aspx

Don't spend that kind of money however without doing some shopping. You can find new or nearly new supercharger head units on Ebay for under $1500 all the time. Again, Rotrex is just one of the many blower brands that will work. You just have to take the time to shop very carefully.

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2002 E55 Stock
Sold my project car: 1993 300E (3.2L) Blower running 0.8 Bar, Re-badged E32 Kompressor, Apexi Piggyback ECU, 310cc injectors, 255LPH Walbro Fuel Pump, Water/methanol injection, Quad SS Exhaust Tips, 3" SS exhaust with high-flow Cat, 17" Monoblocks 225/45/17, Brembo BIG Brake upgrade, Euro-headlamps, clear corners, Euro-tail lamps, keyless entry, CD AMPs Subs etc...

Photos: http://share.shutterfly.com/share/received/welcome.sfly?fid=c20c2dad4373aa89&sid=0CbuXLVo5cMXFg