I posed a similar question, and looking at the code:
The subscription attribute is to be used to allow visitors to be notified when something changes or is added to the object as a child. I guess this will be most useful in forums, and comments with articles as it will create a notification rule for the user who clicks it (not seen in the svn templates yet).
To have free and pay content, you will have to implement roles (access to) and workflow (payment). This is not really in the base distribution, but the provided samples may give you a headstart. The "subscription" feature is probably useful too.
The subtree subscription datatype works as follows. You create a class ( e.g. forum message ), which has this attribute. When a user creates a new message he sees a checkbox. When this checkbox is ticked off and the object is published a notification rule is created for the nodes this object is published in.
Then when someone creates an object under the created object the user will be notified about an update via the notification system.