Here an explanation of the system. It's not exactly your car though.

Did someone tell you it is bad, or are you just going by the code? There are many parts to the system and sometimes the code may not point to the exact part.

 

MERC dealer did a diagnostic, charged 220$ and now they want another 700 to replace this unit.

 

They are referring to the A101 Diagnostic Module that is on the charcoal cannister. They do go bad. I have replaced plenty.

 

It's a small pump that creates a vacuum in the evap system to basically do a self test for leaks. On your car I believe the charcoal cannister is behind the passenger rear fender liner. It's attached to that. Not really that hard to replace unlike the CLA/GLA/GLB/A Class that has it attached to the side of the fuel tank.

 

So it's just a sensor that went bad? The actual evap system is still working?
This testing modular for the evap system, how often does it do a test? I'm assuming every time the car starts?

What do you think something like this should cost to fix/replace at a normal shop?
Do you have any videos or how-to's to do this on my own?

 

There could be a leak in the Diagnostic Module and it leaks vacuum over time. I do not have a video on replacing it. It is fairly easy to replace though. Remove the right rear wheel. Remove the right rear fender liner. Remove the hoses and lines going to the charcoal cannister. Don't break off the lines. Remove the charcoal cannister upwards slightly by releasing the tab behind it. Take off the two or three T25 screws and remove the white diagnostic module and install in the reverse order.