For dopers outside the US I will mention that in the states diesel typical costs about 20 to 30 cents more a gallon than regular gasoline. There was a short period where the price difference was much larger but AFAIK it has stayed pretty constant.
It varies, but I would say that I’ve seen most places selling it for about the price of midgrade. I’ve also been figuring it as about equivalent to gas at 3/4 of the price because of the better mileage (which may be conservative). $3.20 diesel is like $2.40 gas. A bonus is the extended range. The gas and diesel versions of the same car will use the same fuel tank, sized to have adequate range for gas.
A downside is crappy cold weather starting characteristics, and the mileage advantage isn’t there until the thing warms up. Mine has glow plugs, and an idiot light on the dash to indicate they’re warming up. You’re supposed to turn on the ignition and wait for it to go out before starting the car. It goes out almost immediately anyway.
Oh, and another current advantage in the US is a $1300 tax credit, which offsets much of the extra price for the diesel version.