[Ted S]

I've custom built mods like this for sites but since it gets pretty tricky (you have to decide when payments come into play, how to handle members that don't pay, what emails to send, groups to put them in, etc...) I don't see anyone releasing a single common mod that would do this tomorrow.

That said you could use the existing sign up process with some edits to the templates to talk about payment options. Then once someone registers redirect them to the subscriptions page and keep them in a locked down group that just sees a big "You're registration is not complete, please pay to access the site...." message until they do so.