On multiple recent bookings I’ve searched for B and M fares for potential instant upgrades and in every instance “M” was significantly more expensive than “B” and was always or almost always more expensive than buying a paid F ticket.

For an IAH-AUS RT in a couple weeks and I can buy V for $423, B for $507, Z for $533 and M is $1087!

Since 1Ks can get instant upgrades on M fares, I’ve searched for it in addition to B and I don’t recall seeing M less than B on anything I’ve looked at recently. All the flights I’ve looked at originate from IAH but I see this same pattern for multiple destinations and dates.