What kind of content are you providing to the subscribers of the $100/mo. forum? And are you using actual forum software or some other CMS to manage the community?
It's tough to do this stuff in a vacuum. I've got a hipchat room I hang out in regularly that has the same vibe, and there is value in hanging out with smart & motivated people working towards similar goals.
Finally (and currently the biggest part) I have a forum of 20 like-minded people all helping each other out each day. I'm pretty active as well. Just answering questions ranging from landing page feedback, pricing advice, to which audience you should choose.
Unfortunately, just about every forum that could be plugged into one of those "Membership site made easy" Wordpress plugins seemingly got lost in the 90s.