An astronomy forum and selling site that I was on became a "pay to post" site a few years ago, and it dwindled from hundreds to just a handful of users after the announcement, and it was only $12 a year. Formerly it was run on donations, which I did a few. My participation in that forum was already declining, so I chose not to continue. There was another free site that started up a few years before that, and most users went there.
Making a forum a pay site just to post is a certain path to kill a forum.
