You are not charged anything for using the HEWMEN Cell app and there is nothing available to purchase - you are simply contributing your unused CPU capacity to help solve research problems.
As part of your donated processing, there could be some costs associated with your computer's electricity use, internet bandwidth, or mechanical wear and tear, etc., but these types of effects are typically going to be very low.
If you have concerns, you can always change your processing donation limits in the Cell app settings panel. More information on how to adjust Cell's donation settings is available HERE.