To do this cheaply, you need either a billing script that will use paypal IPN, or one that will use the
2checkout service. Both of these allow you to setup recurring monthly payments. To use 2checkout, you have to pay a one off fee, so there are no monthly fees.
One script that I think can do this cheaply is
whois.cart, but there are many others. I mention whois.cart as it is probably the cheapest solution. There is also
phpcoin, which is free, but I am not certain what payment processors it supports as I have not personally used it.
