As title really. When selecting an Action you pay for in Priority Missions, despite the fact in has a cost in credits - you are not charged anything for purchasing it.
These are Actions like: Tomes of Murder, 3 Imperial Guards, Gas Attacks ect - you are not charged for any of them.
EDIT: I tested this (which was absolute hell) to find that it is taken from your balance at the end - you'd never see it unless you just calculated everything at the end - which I did - and it was bang on. Sorry - my bad.