Normally, I'd say visit almost any other forum that has ads appearing on it and click the "Remove Ads" link. that would take you to a page that spells out the costs. But at the moment I can't find a single Nabble forum that is displaying ads - not even those of my own for which I haven't paid to have ads removed.
I think Nabble may have a problem currently.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.