I would pay to stay on the MBs I visit regularly. Some to which I already make contributions. I understand there is a cost borne by the board owners, so if they are providing me with a source of entertainment and good discussion, I would help them bear the cost.
Now as for a maximum fee? Can't call that one, except to say it should be low enough to not prevent well-meaning posters from participating, but high enough to prevent trolls from joining on a whim just to cause havoc.
But by the same token, serious trolls would probably just pay up.
