Good question. I have not yet done as a stand alone treatment, but incorporated this within facials - how many steps depending on length of facial.
Part of your charge can be judged by the area you are in and what the salon/spas around you are generally charging for similar services. You don't want to price too low or too high for the area.
I'm thinking, for the full complete scalp massage $1 a minute - so perhaps $25-$35 range. Where I am it would probably need to be about $30.