RSPB Membership

I am very interested in becoming a member of the RSPB. I know that this might not be the right place to post this message, but I am hoping that someone might be able to help me. I want to join by Direct Debit and I have seen on the website that the money can be taken out of my account on a yearly basis if I wish. The only problem is, I do not know how much it will be. I am on Income Support so any answers would be welcome.