The price of their current form. Of our forms. Our powers are just an extension of ourselves, it's not something that can be gained or lost unless you lose yourself and thus gaining new powers is simply gaining back more of yourself. To reach the realm we are in now, a price must be paid, usually, it's one's humanity what must be left behind. Ironic as it's normally that humanity which drives us to aim for this existence, because of that many forget why they become the way they are now and get lost in boredom and fear.