Do the math. Diesel costs about 15%-20% more than gasoline in my neighborhood, but diesels get 50% to 75% better mileage than comparable gasoline autos.
And this will change. Diesel is only so expensive because it's not as common. Diesel should actually be cheaper than gasoline, since it's easier to refine and you get more of it than gasoline from a barrel of oil. The only problem is that in the cold months the diesel supply competes with the demand for heating oil, which is almost the same thing.
Of course the government has a problem with this similarity because people like to use the low-taxed heating oil in place of high-taxed diesel in their cars (which is why heating oil is dyed red). Just don't get caught pumping from your heating oil tank into your car.