steve,
There a some many types of single seat, with and without vents produced in various material by Ducati, never mind the aftermarket types.Then you have the variable of how complete the unit is, with or without seatpad, latches lights etc.
Unfortunatley E-bay bargains for Ducatis are thin on the ground as too many get carried away with bidding fever.
A bare single seat unit, i.e. without seat pad, lights and latches from an SPS/R not in carbon is gonna be at least 200 plus on E-bay on average, probably a lot more due to the demand.
A bare carbon 998r seat unit is about £520 retail and well over £300-350 on e-bay.
other sources worth checking are aftermarket peeps such as Vimori, QB carbon, Ripsnorting V twin etc, etc,
www.ducatitrader.co.uk? might be worth a look, maybe breakers, keeping an eye on the for sale section on here.
Some official dealers do have clearouts of S/H or old stock from time to time.
It is worth looking beyond e-bay.
You could offset the cost by selling off you existing seat unit. IMHO it s better to keep it then sell off single seat when you evetually sell the bike.
Hope this helps.
Ray