Diesel is about 8% more expensive than petrol where I live. Now assuming 12000 mi/year and 25mpg/33mpg for petrol/diesel, you will save a whopping $1.49 per day. I'll bet the price premium for these new Mercs is way higher than that.
Depends on where you live. Some places, it's cheaper. But there are many other advantages, including increased durability and fuel flexibility. If biodiesel becomes more practical, you're ready for it.
The B-Class diesels gets 50-55 mpg.
The Deisel E class ia about the same price as the Gasoline version. AWD is not available however. The Deisel gets 37mpg Highway as opposed to 27 mpg for the gas version.
I have a friend with an older Benz Deisel, it has >450,000 miles on it.