Cheap isn’t the relevant metric. I’ve run a couple of forums, over the years. It’s entirely possible to customize the software, to make the user experience, for both users and administrators, quite good.

Moderating and managing the forum is the pain.

Also, it can be fairly corrosive to personal mental health.

I recently had an old acquaintance reach out to me, asking me to revive one of my old communities, that was killed by Facebook. This person argued that many of the old community members were now thoroughly sick of Facebook, and would want to go back to the dedicated forum.

I politely declined, and was surprised at how the idea really horrified me.