The MP costs are fine. Part of the reason they are so low compared to what you might expect is that other classes can have easy access to magic to complete the Armory System without making them overpowered.

It was also stated above, MP pools drain fast on NMs, and with party sizes going down to 8 soon, I can see a max of two mages to make room for extra damage to pick up the slack of 7 missing members and the absense of 8 person regimen - I doubt the tank will be able to participate without taking some serious damage, and at least one of the mages needs to stay on healing duty.

Raising MP costs would already push the difficulty limits of what NMs would be with 8 people, assuming they stay as they are. Uraeus will remain fairly easy, but the others might become problematic.