From the FAQ located on the CoS website:
C. Why is there a Membership Fee?
You don’t have to join our organization to consider yourself a Satanist, you only need to recognize yourself in The Satanic Bible and live according to the tenets outlined therein.
We don’t ask people to join—that is your prerogative, we simply supply the information that explains how you can affiliate if you so choose.
A registration fee was established by Anton LaVey himself and is for LIFETIME membership. Many Christian churches charge you a tithe that counts for 10% of your yearly income.
If you are an individual of limited financial resources, you can put the money aside piecemeal, and join when you have saved it up. We’ll still be here, and if it is actually important to you (and one would expect that a religious commitment would have some importance), then you can do it. Look to your other possessions and expenses (most people spend far more than this on general entertainment) and we’re certain you can do this if it means something to you to become a member.
We do not take installment payments.
We have an international organization with people doing work, corresponding with individuals (stamps, paper, computers, email accounts, etc. are not free). Our administrative staff's time is precious—isn’t yours? We are also not altruists. We’re Satanists, so we expect to be compensated for our time and efforts.
If you disagree, then you don’t have to affiliate with us.