Pay to Post - X Credits = X Posts

I know that Pay to Blog has now gone over to Pro Sites. My question is there any other WPMU plugin that supports a feature to allow users to buy credits before they can make a post?

For example, I want to offer a package where a user pays a set fee per month and then they are allowed to make 5 posts that month.

Does Pro Sites support just one site or does it have to be a network of sites?