HJC makes the best CF litewieght helmet. it about $700. as far as cost? how much is your brain and spinal cords worth to you? the lieghter the helmet the lower the forces on your head and neck in a crash. the HJC is pre drilled for the HANS also.
GRM rates some helmets and the HJC is the only one that got rated 10 out of 10.
http://www.hjc-motorsports.com/hx10.htm