Grand vizier halves the meteor cost…star pact uses all the arcane power. So my question is - since star pact is utilizing all of the arcane power, and effectively arcane cost is halved with grand vizier, so the damage is doubled?
straight way is - if 125 arcane power, 30 meteor cost, then 15 is reduced - 110 is left over arcane power, resulting in 2200% dmg boost for star pact.
but is the aforementioned logic applicable? has anyone checked the data?
No.
Using your numbers (125 max AP, 30 cost):
Without Grand Vizier:
125 AP, 30 cost, 95 extra spent.
740 + 20*95 = 2640% weapon damage from impact.
With Grand Vizier resource cost reduction:
125 AP, 15 cost, 110 extra spent.
740 + 20*110 = 2940% weapon damage from impact.
So in this case, the resource cost from Grand Vizier increases the impact damage by 2940/2640 = ~11.4%.
The damage isn’t doubled because Grand Vizier doesn’t double the extra AP spent. It cuts the casting cost in half, which provides a smallish increase to the extra AP spent, and a corresponding smallish increase in the damage.
It does, however, have that other effect that gives 5x meteor damage. That part is far more significant.