There won't be a premium "area" of the forum, per se. People who pay a few bucks a month will just get some premium features. I hope to have this ready by the end of the weekend. I think it'll be a pretty cool addition to the forum, and help me pay the bills without having a bunch of sponsor ads everywhere.
If I install the code that allows people to create private forums of their own, it will cost quite a bit more. I'm not sure if that's going to happen or not, though...I'm weighing the pros and cons of it.