The German auto club ADAC
(German equivalent of AAA) has published total cost of ownership
survey comparing new cars
and trucks equipped with gasoline and diesel
engines. They factored in the initial cost of the vehicles and the cost of fuel based on the mileage of the vehicle with each type of engine. The club looked at 560 pairings of vehicles and the full list is available to club members. There is also a summary chart showing a small selection of the vehicles examined. In most cases the diesel vehicle
was definitely cheaper to own and operate especially as mileage exceeded 9000 miles per year.
However, since many diesels cost more than comparable gasoline engined equivalents, at lower annual driving mileage, the diesel actually worked out to be more expensive. One example from the study is the BMW 320d which costs €1,600 (18%) less to own and operate annually than a comparable 320si. The story, loosely translated by Google translate, is available at the read link.
[Source: ADAC via GreenCarCongress