I'm a member of an open FB group that has about 26,000 members. The owner of the group has a closed group of about 600 that are paid members, I think it is about 200.00 a year to belong to the closed group. The owner does a lot of things for the paid group, more than for the free group and there is still a lot of member contributions in the paid group.
Not a bad way to make a few dollars!

Mack