Skills and spells now use mp_cost, health_cost and vitality_cost instead of just cost. BE gui shows all cost when you select a spell. Also, if cost is not an integer, it serves as a relative value, ie health_cost=0.5 means 50% of max health.
You should add the same tooltips for normal attacks too btw, atm it only shows them for spells. I added a small vitality cost to them, so it's kinda important.
I capped health cost at 90% just in case. If 100% is used, the character dies, but BE doesn't clear him/her out, so additional coding will be needed for it if we'll have suicidal skills.