It is not just the initial higher cost of the diesel that gives it a higher resale value, but vehicles with diesel engines depreciate at a lower "rate". You can read this in the attached article where the University of Michigan's Transportation Research Institute (UMTRI) spells this out in the attached article:
http://autos.aol.com/article/diesel-cheaper-gasoline/
In summary they say that the vehicles they tested saved between 10 - 39% in depreciation costs alone over there gas powered counterparts.