Hero spell cost.

I downloaded a small mod that adds new abilities for Kha-Beleth. One of them is a large wave that damages all non-demon units. I tweaked this ability a bit, reducing the damage and the damage increase from magic, and added a mana cost (in the original, the ability is free, but I want it to cost mana). I set the cost to 4 mana per use, and it seems to be working; the damage changes, and the ability costs 4 mana in the spellbook. However, when Kha-Beleth casts it, the mana isn't deducted. How can I make the ability consume mana?