At $51,775, the E350 BlueTec runs $1500 more than its gas-powered sibling. Our test car added $6450 via the Premium Package 2 with its navigation, Harman/Kardon audio, HID headlamps, and heated seats, among much else (the Sport package on our car is a no-cost option). But nothing approaching the full cost of the BlueTec system’s hardware appears to be passed along to the consumer, and fuel savings promise to be significant if diesel prices remain close to those of gasoline. About that urea, though: If you’re not planning on harvesting and refining it yourself, expect to spend about $1500 on the stuff for every 100,000 miles you drive, thereby negating a large chunk of the fuel savings. Note that we said every 100,000 miles. The E-class diesel may be more expensive to buy and just as expensive to run overall, but it will keep going until you croak. For this reason, Mercedes diesel loyalists tend toward the evangelical, and the diesel version traditionally holds its value better than any other E-class.