From my own perspective I'd already estimated the cost, based, as A & C has said, on the cost of employing new staff divided by the number of aircraft on their books.
The villains in this piece are EASA and the CAA in introducing the additional burden for no good reason.
It leads to some silly elements
Prior to Part M, I held and maintained the logbooks, my tech log had a countdown of hours to the 50 hour check and the annual.
Now I'm supposed to leave the books with the CAMO and to email them monthly the flight hours so they can update the logbooks and remind me the 50 hours are due. I tell them when I am going to do it, under pilot maintenance, so they send me worksheets. I send them back and get by return a release to service certificate.
In the past it was recorded once in the logbooks, but now we are funding the Royal Mail. I can't blame the CAMO but rather the way the CAA insist in their QA procedures.
Part M was designed along the assumption that we operate as airlines and have easy access to our maintenance organisations. A lot of us don't.
And if they ever get round (as they have said they want to do) to requiring the microlight and Permit fleet to operate under Part M rules, they'll have a riot on their hands.