John,
Have a look at
http://www.tecmoto.co.uk/ for quick release sprocket kits.
They have the carrier for 120 and then you can buy different sized sprockets for 36 each. The difference being that it's much easier to change the sprocket as you don't have to disassemble the hub and cush drives.
TecMoto also do an STM replacement sprocket and carrier - a complete unit like the Ducati one - at 86 quid.
I don't know how much a replacement Ducati one would be, but a quick phonecall to your local dealer will tell you that!