Many years ago I remember reading that, in general, a diesel engine will get about 33% better fuel mileage when compared to a COMPARABLE gasoline engined vehicle.
It's a little difficult to do an exact comparison, but I do remember way back when VW was selling both a gas and diesel Rabbit. As I recall, the diesel version did get about 33% better fuel economy - my 5-speed diesel version would get an honest 60 mpg at 60 mph. So I figure if diesel fuel is no more then 33% more expensive, then I come out ahead by driving a diesel engined vehicle.
Tim