Well i have not done it with a agency girl but have done it a few times with a parlour girl. The way i have done it is just ask her when in the room after all she can only say no. We have split the difference ie if at the parlour you pay £100 and the girls take is £50 you pay her £75 and everone is a winner ,well apart from the parlour but who cares about them.With a agency girl i would think you would get a bigger saving but i would not ask on a 1st meet.