Valve adjustments on Ducs are not easy, so some dealers may try to charge a lot more to cover their "aggrivation" factor.
I have a two-valve 944 (ST2)... the actual adjustment process for 4 valves is about 1 1/2 hrs, including shims, winkler caps, gaskets, replacing cam belts (every 6 k without fail or else) and buttoning her up...and another 1-1 1/2 hr to replace plastics. The rest of the day is spent doing all the other stuff on the PM list: Removing and replacing the stator locknut and washer, retorquing with locktite, retorquing the clutch and basket, checking motormounts, brakes, wheel bearings, clutch and brake master and slave cylinders.....if you don't do this stuff on a Duc, your gonna "PAY" for it later, either to a shop or in poor performance and breakdowns. If you do do it at the regular intervals, they are as reliable as any other bike. Average cost in parts every 6K is about 60-70 bucks...oil, filters, gaskets, plugs belts and shims....4 valve heads are a little more.
If you pay a shop to do it, it's pretty much a full day to a day and a half job (if they do EVERYTHING they are supposed to)...that's where all the money comes in.