Any good system will have these features:
1) Encourage a growing membership rather than pushing them away with hoops to jump.
2) Will recognize that in any group, the 10-15% will support the whole.
3) Be easy to administer by those responsible for maintaining the site.
I (along with a cadre of very helpful folks) run another motorcycle site, and we've chosen to not run ads (just to keep anything commercial from intruding on the experience) and rely on donations, often 1-2/year. We'll typically raise $US4,000-6,000 with each mention, and that takes us a long way.
What members get for this is simply inclusion as a "Friend of the Site." (And I'm behind in updating it at the moment.) There are no levels or amounts--you're either on that list or you aren't. Very simple to maintain.
We get contributions of $10-$500, with probably $30-40 covering most of them.
Sponsoring a forum is something that isn't on many people's radar. It's not that they are cheap bastards but rather that they just aren't used to the idea or even understand what goes into an active one like what you have here.
Others have life circumstances that don't give them much extra discretionary income.
Others think they'll get special treatment because of their support--those we simply return.
Anyway, I'd recommend voluntary with one or two "fund drives" a year. Be prepared to get several hundred who are willing to "carry" the site. Give them simple recognition. Go back to riding.