Hi chaps,
I don't think people should think too little of the 96% of people who have never donated money to the site. To be fair, I think many people would simply believe, if they even give it any thought at all, that the site was adequately funded by other means as most forums possibly are.
The 'donation' option has been a user's free choice and clearly most people have chosen to use the site for free, myself included.
Subscription to the site as a way of contributing and therefore helping to keep the site alive is a sensible option and I hope you get a good response that really makes a difference to you chaps who run it.
As someone who has not only sold a lot of gear via the site but has also had bass students who have come to me for lessons having seen my teaching post on the forum, a £20 annual fee seems more than fair.
Good luck chaps!
All the best,
Steve