As one who would be willing to pay a fee to post on PB, I agree with George that charging all members a mandatory fee would radically change the site. I believe that the two tier membership idea is a fairly common one so it must be somewhat successful.
One thing to consider is to include a DONATE key on the main page as many websites do. I've made a donation at The Forestry Forum using that function.
One advantage that PB has over TTF is that PB has products to sell. The magazine, books, videos, etc. could be promoted more to help pay the bills. Perhaps an icon could be included by a member's name when they are a PB magizine subscriber, or have bought books or made donations. This might serve as a subtle but constant reminder that the forum needs money to stay viable.
There is also the pledge drive method like the one used by National Public Radio. I don't exactly enjoy that time of the year on NPR, but with a forum you could still provide the same content while adding a thread that promotes donations and tracks pledges towards the published goal. This could utilize a "sticky" thread which is one that is always at the top of the section it's in. The title of the thread could be edited regularly to display the progress toward the monetary goal so that it would stay in everyone's mind until the total was reached. At that point the thread could be deleted until next year. NPR gives both "gifts" and public recognition as perks to contributors.
I think there is an advantage in keeping the forum free of mandatory fees and allowing people to donate, with gentle nudging of course.
PS. I'm a PB subscriber and I own Perfect Pond. I encourage anyone who hasn't supported PB yet to do so now. There is no need to wait for a system to be installed to support your favorite forum.
