Diesel isn't that cost efficient actually, even if you drive a lot. Diesel are more expensive to maintain due to regulations, plus diesel cars tend to be more expensive their petrol counterparts. Moreover, there is depreciation, which tend to be worse for diesel cars, because they are valued less on the second hand market (fairly or unfairly).

On average, you should expect to drive at least 150.000+ km before you've broken even on a diesel car with its petrol counterpart.

People only look at fuel consumption and fuel prices, but maintenance and depreciation are actually one's biggest loss on a car purchase. And in general petrol comes out on top.