I hear what ypu're saying, but in my experience people are reluctant to pay for anything if there is a free alternative. To be fair, this isn't the only sketchup forum. Having said that, why not look into deals with advertising on the forum?
Another forum I frequent has various levels of membership, not that it has restricted areas, however if you want to sell anything that is new you have to be registered as a dealer (subscription fee). They also have deals with various companies for advertising on the pages (no pop-ups or silly stuff, info is there but not in your face). Using the same concept could work here and help fund the website / forum.
Also wouldn't hurt to upgrade the board software to the latest stuff that is a little more attractive... π