Diesel cars and vans usually come with a larger price tag than the equivalent petrol and also diesel fuel is often more to buy than petrol. This is because diesel cars tend to do more miles to the gallon than petrol and therefore push the purchase price up. Also diesel cars tend to last longer, i.e. many diesel cars will easily do 200,000 miles plus with no problem where as a petrol car is often coming to the end of its life at this stage.
Diesel cars cost more to make (on average) than petrol equivalent and have been designed to last longer, although this is not so much the case with new cars of today.
Although diesel has predominately always been the more expensive fuel, only in the last few days have we started to see some garages selling diesel at the same price or even lower than petrol.