According to this website Daily Fuel Gauge Report--national, state and local average prices for gasoline, diesel and E-85. the current national average price of diesel is almost 40 cents more per gallon than premium gas.

Premium is $3.48 and diesel is $3.84. Without trying to figure out the exact % difference the diesel is roughly 10% more expensive than premium.

While I agree that X5M is a fuel guzzler (though no gas guzzler tax!), I am doubtful that the M diesel with 380hp is going to be a fuel miser and will certainly consume more diesel fuel than the 35d.

All this brings me back to the question of what is the niche for this vehicle?
Is it the fuel conscious driver? If so, then this vehicle is not exactly an answer. With diesel being more expensive in the US and this M diesel car probably not getting a great mileage, then who did BMW marketing group ask or poll for the need of this vehicle?

Thinking out loud on the logic of this creation...