I own a Ducati Monster S4R, with the 4v 996 motor, and have owned a Monster 620. I can tell you that owning a Ducati isn't as expensive as people make it out to sound. Cost of ownwership is largely affected by how mechanically inclined you are and how many dealers are in your area. The reason they get a bum rap is because the valve service interval is every 6000 miles. Far more often than other bikes. But neither of my bikes ever gave me a lick of trouble. I use my current Monster as my daily driver, weekend back road burner, and take it to the track once a month. So they aren't just for looking good in your garage.
Call some local shops and ask how much they charge for valve service. It should tell you all you need to know about cost of ownership. The rest of the service is the same as other bikes. Oil change, brake pads, tires, chain. Typical wearable items you have with any bike. The valve service is the only thing you'd be likely to have a dealer do. Lots of owners do their own valves. People get scared because the desmo valvetrain is seen as a mystery. Monster owners with more than 1 bike say the Monster valves are actually pretty easy to adjust.
As for the 750 you are looking at check the service record. Becuase Ducati's have such a bad rep people document their service very well because they know when it comes time to sell the perspective buyer will want to see the service (read - valve adjustment) has been done every 6000 miles. So check to see the owner has papers showing the service interval. Also, and this might help with negotiations, since the bike has close to 12,000 miles the next service will be due soon. The belts get changed at the 12,000 mile mark. So it'll be due for valves and belts. If they aren't done already get him to drop the price to cover it.
I'm not going to say owning a Ducati is cheap. But it isn't the money pit people make it out to be. And they are very reliable. It all depends on how the owner took care of it. Which is the same when buying any used motorcycle.
Check out
www.ducatimonster.org That is our version of SVRider for all things Monster. There will 750 owners with info directly related to the 750 as well as a good price for a used one.
Incidentally, I just picked up an SV over the weekend. Still have my Ducati. The one area where it is expensive is the price tag. I couldn't keep risking a $14,000 bike at the local trackdays. So I picked up the SV for track duty. I totally dig it....