Cue neglegence on my part, i honestly didnt look about MP cost vs Holy.
Honestly Grav is fine as is, it was just a thematical "surely it should do x"
from a balance point thought if they were to add heavy as an effect then id definately expect it to cost more MP. as to whether it would cost the same as Holy, not sure, personally i find Stun to be a more potent up front effect vs heavy, so maybe 5% less MP cost than Holy?
still, good call; if i ever make towards wanting AST changes ill make double sure to cross reference costs etc