2011 E350 Bluetec Diesel at 130,000 miles got the oil leak
 

finally. $1 o-ring between the engine and oil cooler, $4000 in labor.

Sold the car, Honda Accord Hybrid incoming.

It was a decent run, but I am all done with M-B

I heard you can bargain labor down to $1600, but what's sold it is sold.
I hope you like Honda. My ski buddy drive Hondas for decade.
He junked first one when with over 300,000 miles would not pass the smog.
Than he bought new one and when his was about 3 yo, I drove 10 yo MB diesel with 300,000 miles. He could not stop prizing my MB for smooth drive comparing to his Honda.
But fact is that 2nd Honda, now with 260k miles cost him only few hundred$$ for clutch replacement.
Good luck with Hybrid. Come back and report if you can match the 60 mpg my E250 Bluetec can make. 40 mpg in city driving.
How much you've got for Bluetec anyway?

That amount of labor is outrageous. Is this a common issue for the diesel?

I had a chance to drive the Accord Touring 2.0T and Hybrid last month. Accords have always been solid cars but they finally caught up on the tech. The Touring trim feels a lot more expensive than it is IMO.

It depends. I did the job on 6l Ford Powerstroke and labor goes in tens of hr, not to mention that dealers like to lift the cab for such jobs.
Than did not do it on 250 Bluetec, but I think I see oil cooler bolted to lower frame.
10 seconds to access.

The oil cooler o ring seals are a common failure for the early OM642 V6 diesel. The new o rings are a different (Viton?) material and should last a lifetime.
I replaced mine at 42K miles, and it took me 20 hours of labor. If you estimate $150/hour it saved me $3000. About $200 in new parts to do the work. Dealer markup will make this $4000.
Many of these hours are cleaning the intake carbon and prepping parts for re-install. Most shops would skip this.
About a 6/10 in terms of difficulty, so not for the shade tree mechanic, but higher skilled wrench.

There are not too many other 'known' issues with the 2011 W212 Bluetec, except for the intake carbon buildup, so I think I have most of the bugs out.
I would also recommend an SDS for any MB that is E class or higher. They pay for themselves with the first problem you have. Consider it a cost of ownership.

I'm happy with the car and hope to have it for 10 more years. Best of luck to you in the future.