Hello. If you're handy with some tools, you can do it yourself and save yourself the labor charge, and you'll know it was done the right way. I had the same thing happen to me and it's a DIY job if you got some time and patience.
Go to www.fcpeuro.com and buy the kit from there, according to your vehicle specifications. You should change out your serpentine belt, both pulleys, and the tensioner.
Use the link for reference only... Mercedes Drive Belt Kit OM642 - Continental 521844
Serpentine belt replacement for your reference...
Lastly, if you're asking what the labor rate is, then all shops have their own set price. Some will charge more and some less, but what matters the most is that the shop doing the job, knows how to work on a MB. Google some local shops and call them and ask directly about their labor rate. Most time they will go with what the computer say, since it's a set timeframe for the work performed, so what that means they will quote you 5hrs or 6hrs or 8hrs for the labor job, plus cost of parts.
Some shops will not let you bring your own parts as they're also making parts on their parts from their parts supplier. Take your pick and decide. GL.
