I don't know anything about your monovalve, but I looked it up on Fastlane using the button above. They list the thing for $400 or $253 depending on which you need. Still expensive compared to a 300E.
It seems to me that early 90's S-classes can be really expensive. There is a known problem with the wiring harness, too, that can set you back thousands. We are all used to climate control problems with our cars, but if the entire electrical system can fail as well, you have to be thinking production defect. The S-classes are typically out of warranty now so many owners are starting to feel as you do.
I suggest that you post a message something like "Poll: S-Class problems", track people's troubles and start organizing a class action response. Mercedes has to know that there are big problems with your car, they just won't want to admit it.

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VR
1967 250SL