strangely, with so many free options nowadays, paid sites often just end on facebook (something i gave up years ago). too much admin and moderation incase some snowflake gets offended.
the ads are a option to free site platforms, best option is to choose weather they annoy you enough. (blocker options)
fee's for members and special areas is a tad dated.
i'm on a few motorbike forums(as i ride bikes), as soon as i seen members fees and areas pop up, you could see the forum dwindle.
paid forums like that are often very high traffic. again, the supplier charges more as the traffic increases.