Casting Cost = base vigor cost + drum vigor cost - esprit reduction
spell base vigor   formula
Esprit vigor reduction = skill + 2   <-- formula changes at higher levels!!!
heal 100 5 * skill + 11
heal balm 220 22/3 * skill + 97/3
rest. zone 400 6.4 * skill + 29
life boon 340 11 * skill + 49
Revit 160 8.14 * skill + 192
Harm 26?
Lightning 50
Charged Air 150
Eye of Storm 80
Thunder Clap 110