Diesel costs more than regular gas, but a few cents per lire less than the highest octane gasoline (RON 91 in Atlantic Canada). It's been almost 2 decades since I owned a car that drank anything other than 'high-test'.
By many reports, the E350 Bluetec easily gets high 30's per US gallon, significantly better (at least 10 mpg) than the gasoline engine. This is one instance where a car actually betters its EPA rating.
2 year-old Bluetec's sell pretty much at the same price as their gasoline sisters. New, they ran about $1200 more.
The E350 Bluetec is almost unheard of here in Atlantic Canada, because it's not been available in North America in 4Matic (AWD). In snow climates, the dealers stock the AWD. With stability control and snow tires, I'm happy with RWD, and much prefer it over FWD where, if you lose drive-wheel traction, you lose steering.
For 2014, the Bluetec diesel 6 cylinder 3 litre has been replaced with a 4 cylinder 2.5 litre. More efficient, but not as smooth, and less power..