I have 2 Benzes and have a very good indie. Everytime he has put in parts I check pricing here on the forum. He jhas been right on the money. He buys from a Mercedes dealer that gives him a 20% discount.
On the few occasions I have asked to supply my parts, I have him add the 20% that he would NORMALLY make on to the bill in the form of a little more labor. You are a cheapskate if you expect your mechanic to lose the little he would make so you can save a few dollars.
Anziani

87 300E 203K (for sale)
95 E420 112K