With regard to Canadian's "M" fares -- they are the "dirt cheap" fares, i.e. Web Specials, promotional, seed fares, etc. Of late, M fares, as well as K fares, have been excluded from some double and triple points promotions. But you are right about the "B" fares. They can sometimes be higher than a Y1 or Y2 (Y class) fare.

Regards,

Ken Hamer