There is no "M High Flex" or "M Low Flex", there is just M Flex, and M Standard, and M Basic (unless I misunderstand the structure?) and M is expensive in terms of discounted fares. As far as I know it is the most expensive discounted fare available, so yes, M is expensive, regardless of Flex, Standard or Basic.

M has traditionally been a high category fare class, the expectations around flexibility and "refund-ability" go beyond how AC has manipulated their branding of the fares. Just like if I purchase a B fare, similarly, if I purchase a M fare I expect certain things; that's all I'm trying to point out here. Going back to my original point, the ability to earn AQM's is not equivalent to PQM's, for a 1K, or even as a SE flying on UA. Some people on this forum don't get to choose their fares for business, they are chosen for them. If you are in the unfortunate situation of having to purchase the lowest available fare, your earnings potential on AC is likely 50% less than UA in the North American market.