Time for a game. See how much money can be spent on two parts for a 380SL. The idle speed motor (most likely the other part that gets dirty under the hood) MB part # 000 141 12 25 and the Idle Speed Controller MB part # 002 545 33 32. I will tell you that current MB list is $428 for the idle motor/valve and $310 for the controller that was the fix here (did you say $400).
I have never looked a part up in partsshop but I Imagine that these parts can be looked up by number there. Don't look at rebuilts if they have them. New parts will be VDO and willnot have the MB # on them but will be the exactly same part (I'm fairly certain). See what they ask for these parts. I haven't looked but I did look at one of our mutual suppliers for my cost. It might be embarassing to see the difference in what they charge and the prices quoted here.
Bosch Master, ASE Master, L1
33 years MB technician