The MP costs are perfectly fine. It is already no longer viable to sufficiently heal yourself on a DoW in a prolonged fight, effectively reducing solo-ability of mobs that were not meant to be soloed.

Also it makes it so mages can't just spam their heals and have to think more about MP management.

They did not overshoot it. They got it just right. Besides, once they implement the ability to cast spells while in passive mode, which they confirmed in the 1.18 patch notes, mages will be regenerating MP constantly since they won't have to go active. If you reduce MP costs along with this, we go right back to square one.