I reckon £4500 is not far off the trade in price you'd get at a dealer. In fact, a DSCer asked about trading a 2003 749 for a "perviously enjoyed" MV Brutale just a couple of days ago and was offered £4,800 I think.
Having said all that (as you said 'bikes' not just Ducatis), my lad wanted to go from 50cc to 125cc and was offered £850 trade in on his Supermotard against a 125 (chinese) Supermotard. We sold it on eBay last week for £1200 so he is smiling.