The arrangement is determined by the owner of the Master account.  Before following the Master account a Slave account must agree to the terms.  Typically if the PAMM trades result in losses then there is no fee due to the, "Master."  It is usually a function of positive returns [profits].  

So, just as an arbitrary example if I were the Master Account holder I could set the conditions such that Slave Accounts owe me 0.5% of profits made while Slaved to my account.  If trades lose money profits are negative so I am not owed anything.  If profits in your account from following my trades are 1 BTC, I will automatically receive 0.005 of that and you will keep 0.995.

You will have to check the specific conditions set by the Master before opting into their account as a Slave.