Some pay X amount per round lot traded, subscription, % of spread, fixed cost per follower or combination of methods (eg. % of subscription + amount per lot). As hedgescalp says, it depends. Just read the terms of the system you choose, the information is usually contained within. Look out for catches too, like payment only made after X lots traded, minimum followers required before payment is activated, etc.
For every loss there should be at least an equal and opposite profit.