You don't buy a diesel to save money... You buy a diesel because of the pulling power it offers. I came from a 2002 2500HD 6.0 gasser to my current 2012 Dmax and the towing experience is nigh/day especially through the mountains. Yes I do get ~20% overall better mileage with the diesel but after you factor things like up front cost, additional oil, DEF, fuel filters, etc I doubt its going to save me any money in the long run. I sse too many people try to crunch numbers thinking they are going to save money with a diesel when its really not the case.
That said once you move from a gas to diesel and get a taste of how it tows you will probably never want to go back.