No need to justify the cost of a diesel. If you can afford the higher upfront cost and want one, buy one. If you can't afford a bit more up front or don't want one, don't get it. Same deal as with luxury packages and trim levels, except on the diesel you actually get some of your money back when you sell it.
The difference in total cost of ownership between gas and diesel is immaterial when you add up what you'll spend on fuel, maintenance, tires, aftermarket add-ons, repairs, depreciation and etc over the life of the vehicle.