I paid £32 delivered, off ebay.
Took a bit of a flyer though as the pics didn't show the fast Idle button very clearly .. I think its the right one though.
For that, I got the full works, ie twistgrip, housing and cable.
Seemed a decent price compared to others so I nabbed it, but ...
I nearly waited and had a bid on this ...
http://www.ebay.co.uk/itm/DUCATI-916...wAAOSwcBhWWzLm
Definitely can't see the fast idle button on this one though, so might be worth asking before bidding.
Note, as Tim says, its a single cable throttle ... which is probably what you're after, but just saying in case.
Also it may have a faster action, though then again maybe not compared to your original .. my original is slower (and therefore lighter too) though.
Mine should be here in a day or two, in case you want to compare notes again before bidding ....?
ps. I haven't forgotten that I owe you a fiver for postage by the way.
I've been waiting while trying to find a suitable trinket to include as a thank-you before sending it.