Sorry, I don't actively collect postal stationery.
The classic reference work will be the Higgins and Gage World Postal Stationery Catalogue. It's about 20 volumes (ABC order), but I'm not sure which volume the M's are in. The last printing that I know of was in the 1980's, so the prices will be out of date.
A very rough rule of thumb, is that when a design appears as both stamp AND stationery, the stamp version is usually worth more. But there are lots of exceptions. It's more of a demand issue, rather than quantity.
Regarding postal stationery of Monaco, the big bucks are when you have a stamp tied by cancel onto postal stationery (stamp used to pay for higher rate). These whole "covers" in good condition and right combination will easily fetch 3 figures at the proper auctions.
As a general rule, if you see a 19th Century whole cover consisting of a stamp tied onto stationery -- don't cut and soak, but keep it intact and check into the value first.

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