Assuming the dealer diagnosed the problem correctly as the head unit, I would suggest replacing it yourself with a used one. It's surprisingly easy to do it yourself - 4 (Torx) screws. See my recent thread on this, for the W-211 E-class.
There's even a video showing how to do it. And you don't even really need to disconnect the little control panel below the radio the way they do - just set it to the side. (But use some paper towels to keep the dangling wires from leaving some black marks on your dashboard). If you're reluctant to do this yourself, you could probably still buy the part yourself, and then take it to an independent shop that handles Mercedes and have them do it for just the labor charges. (A friendly Mercedes dealer might do it as well). Just ask them first.
My local LKQ parts warehouse had my unit without Nav, and your COMAND unit, for the same $150 price, and you don't even need to exchange your old one (which is good, in case you find that it's the audio gateway that's the problem instead). Find the closest warehouse to you at the LKQ site. LKQ has their inventory on line, so you can see what they really have in stock. (They also have "pick your own" yards if you have the taste for that, but the warehouses sell you a unit in a box, tested, with a 6-month warranty.) This company mainly supplies parts to insurance company repair shops and professional shops - it's the largest in that business. You can find other similar companies through car-part.com, which also shows actual inventory in many cases.
The W211 (E class 2003 - 2009), audio systems were all the same except the last few years. See the audio/telematics forums on benzworld.org . There's some good FAQ type material in the "sticky" postings.
A local Mercedes dealer wanted about $1741 for my remanufactured unit, and about $360 to install it (which seems high for an easy job). The Mercedes dealer parts guy there said that almost no one really buys a new unit.
The fact that the radio starts up, but then shuts down, suggests that there is a problem with one of the networks used by the head unit. It's their designed behavior to shut down with a network failure. (There's a CAN network to communicate with the steering wheel buttons, and the MOST network to communicate with the audio gateway in the trunk, which is in reality the amplifier and, for many units the tuner as well.). Any unit on these networks will take down the other units, so a MOST network failure could be in the head unit, audio gateway, CD changer, or, less likely, in a connector.
I had paid the dealer to do the diagnosis (about $125), and they diagnosed my problem as the head unit. I think the dealer has enough test equipment and expertise that they can pin down the source of the trouble, so you can probably believe them if they specifically told you it was the head unit.
An interesting side effect of my head unit not working, was that I couldn't open the secret compartment below the head unit. That's because it also needed a working network, which stopped working when my head unit failed. The instrument cluster display also, after waiting for a timeout period, showed no radio or telephone available.
BTW, in other postings about Mercedes audio problems, a lot of people reported that the problem started out intermittently before total failure.