Rather than just question the viability of the premium account idea without offering alternatives, I've taken a look at how other boards handles the same issues without resorting to some sort of multi-tiered poster structure.

The first thing they do is employ Amazon and Google Ads, and make big drives to get people to order books through those ads (they get a cut of them) rather than just going to the sites direct. You have a lot of people reviewing books on this site, it should be relatively straightforward to set up a link on those reviews to the respective Amazon pages, and if people like the sound of the book they can order it that way and RAFO gets a cut of the cash. Since RAFO has provided a service in making them aware of the book, that works out. Obviously, you can also encourage the WHEEL OF TIME fanbase to order TOWERS OF MIDNIGHT via an Amazon link on the site as well.

Secondly, is the good old-fashioned pledge drive, maybe mixed in with competitions and raffles. Whenever other sites' hosting fees are coming up and they haven't made enough money via advertising, they simply say, "We need £xxx, we've provided you with countless hours of entertainment/job-endangering time-wasting, help us out," and put up a PayPal account. When they reach the amount needed, they take it down, say thanks, and sometimes throw in prizes or some sort of raffle arrangement so those who contribute get a chance of winning some books or other gear.

Thirdly, sponsorship. This is harder to get, but as an SFF-related website with several thousand members, it's possible that at least one or two publishers might consider some sort of deal in return for advertising their books on the site in a relatively non-obtrusive way. £6 a month to get their names in front of a couple of thousand book-buyers isn't a lot to a company like Tor or Orbit, especially considering the site's long association (through its direct ancestor, Wotmania, which I would recommend playing up, as Wotmania was very well-regarded by some publishers) with the WHEEL OF TIME web presence.

Just some ideas. Other sites use them to pay for much higher web hosting fees than RAFO has, so they do work.