The fuel and cost savings from owning a diesel will be small. When you look at it as a percentage of acquisition cost, almost trivial. I've heard some stories that this will change as our involvement in 3 wars diminishes--not sure how much I believe that.
Diesel vehicles are about much more than cost savings. Perhaps the biggest reason we have them in North America at all is because consumers wanted a choice. Diesels will not be a profitable segment in the near term, or possibly even in the medium term.
Diesels do provide a good interim step as the world weans itself away from fossil fuels. And in a world where governments are beginning to legislate how efficient passenger vehicles have to be, diesels serve a compelling purpose for manufacturers looking to increase their corporate fuel economy averages.