I am trying to figure out what the "payback" cost is for a Mercedes Sprinter diesel. When you add the additional purchase price plus the increased cost of fuel and blue deff I don't think you gain enough unless you are driving a lot of miles per year. Diesel fuel is always more costly than gas. I really do not know what the true fuel milage is on a 24-27 gas vs diesel but I am guessing 10-12 vs 15-16.
You'll likely find that there is no "payback" on the extra cost for a Sprinter based class B.
Resale will be higher, fuel mileage should be a little higher (we get about 17 mpg in our Roadtrek) but diesel is not significantly cheaper than gas to make up the up front cost difference.
One thing to note is depending on what part of the country you're in the difference in diesel vs gas can be significant. Some places it's cheaper, some more expensive.
As far as the cost of DEF, I've added a gallon ($4.00) in the 4000 miles we've had the van.
The main advantage of the Sprinter is the torque the engine produces gets you over the mountains and speeds you down the long boring interstates (when you have to do that sort of thing) without straining the engine. It's also rated to tow up to 7000 lbs (depending on cargo load) so if I choose I can take our Jeep or a boat with us.
Good luck with your choice!