Refer to mana cost of ability being cast

Is there any way to refer to the mana cost of the ability being cast? I need it for my spell that is kinda like a mana flare targeted spell.

It uses attachedvariables to save the level/damagesource to the target then when they cast an ability, it uses the damagesource to "damage" the target (based on level) for however much mana they just spent casting a spell. The trouble is, I have no way to get the amount of mana they just spent! I appreciate anyone's help in advance thx :D

The action "unit is beginning the effect of an ability" only returns the casters current mana (before the ability is cast) and I need to return the real mana AFTER the ability is cast... Anyone got a function for this? Something like

function Hero_mana_after_ability_cast takes abilitybeingcast x returns real?

Actually, looking at Vex's spells, his "blood mana" spell does exactly what I need. I simply changed the AOE to 0 to make it a single target spell (It's ability is acid bomb).

I still would like to know the specific of how to do it, vex uses his attached variables and caster system which I obviously don't know from beginning to end so his code looks a little funky to me because I don't know all the functions by heart.

Mana before - mana after?