Why pay, if there is something available free of charge?
Your forum would need to be unique if you wuold want to keep your users.
That's probably the most important consideration. If you have something that is truly unique, and that only you can provide
, then people might pay.
Maybe some of your users would even start a rival forum as a protest for your paid one.
I have seen this happen.
When you can get phpBB for free, and a domain name for under $10, and hosting for next to nothing (for a smaller board) there's not a big barrier to entry for someone that wants to start a competing board.
There's one board that I go to (not phpBB, but that doesn't really matter for this discussion) where you have to pay to join. But what you're paying for is a magazine subscription, rather than board membership. It's just that they don't let you join the board unless you subscribe to the magazine. The discussion board can be viewed as a marketing effort to sell magazine subscriptions, I guess.
Let me turn the question around and ask, why are you considering going the 'paid membership' route? Are you trying to make money? (for the record, there's nothing wrong with that.
) Are you trying to cover hosting costs? Have you experimented with advertising? Google Adsense can work fairly well, if you have the traffic to support it.