I don't agree with charging people by the post as its the posts them selves that make a Forum what it is - without them RIBnet would not be the fountain of all boating knowledge that it is.
I am perfectly happy to contribute but charging per post would only discourage people from parting with their knowledge and experience.
Here's a suggestion that seems to work on another Forum I use www.xda-developers.com
They use a system of donations and for every £10 you donate you get a little gold * next to your name when you post.
I don't actually know how much revenue they generate from this system but they have managed to keep the site going with out resorting to banner adds for years now...