The point is not to charge for entry that is a hard thing to do. What one could do is charge for addons on the site like Photopost/Reviewpost. Different Usergroups access that gets you into a set of forums that have some extra features.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.