There about £50-£60 from Germany, it's just a clip that clips into the throttle tube, i think I still have one for a twin cam, also fits single cam, the only difference is the outer throttle casing is slightly different on the single cam bikes.
Ive fitted these to all my GS and R bikes and they do make a noticeable difference.
I'll stick a picture up tomorrow, I would happily sell it.