I am just a little surprised that MB would not have protected such an expensive unit a little better than that. How much would a little extra insulation cost?
Hopefully your control unit is fine. You are probably not getting ripped off, but I will attest to the $850 price, since that was the same that I got quoted from the dealer.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car