You dont need to pay for anything. It works this way - the boy sperm are fast and the girl sperm are slow - if you bd on the day of ovulation then the fast boys hit the egg first = ie chances of a boy, if you bd before ovualtion the the boys die before the egg is released and the slow girls then hit the egg - ie a girl.
it's all chance though at the end of the day, but thats how you up your chances of having a girl or boy.