While the Internet is populated with free forums, there is also the possibility to create a private one where members need to pay a single or recurring fee to join.
SEO Blackhat charges $100 monthly from its members, and they have thousands of them. Obviously in order to charge such a price for a forum membership you need to provide real value for the members (e.g., secret techniques, tools, and so on).
Performancing also launched a private forum recently, focused on the networking aspect. It is called The Hive, and the monthly cost is $10.
These are just two examples. There are many possibilities to create a private and profitable forum, you just need to find an appealing angle that will make it worth for the members.
List of popular forum software: