I did not re-read the below so don't now how much will be helpful.
For the last year at least 2 persons have said the Dealer has a price of aroun $75 for the Governor Diaphragm.

Governor Diaphragm Test
http://www.peachparts.com/shopforum/diesel-discussion/313733-74-240d-grey-smoke-questions.html

has pneumatic governor tests
75 240D injection pump oil consumption - Benzworld.org - Mercedes-Benz Discussion Forum

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84 300D, 82 Volvo 244Gl Diesel