If your members pay by e-check/ACH/paper check, there is no cost (0% processing fee). For any card payments, there is a 3.5% transaction fee. These transaction fees are taken out after the member submits payment and before they are transferred to the chapter.
As an example of how processing fees are taken from payments, if a member pays $100 by credit card, greekbill will take $3.50, and $96.50 will be transferred to your chapter.