Hmmm, ... well, I finally have a chance to catch up on the forum.
Short answer: 20-25$/year. That's the same as an annual subscription to a mag. I'm on a couple other forums which are double that, but they include coupons for equipment (camera stuff).
Long answer:
As Betty said, ... it's priceless ... 1) Because of the quality of information and 2) the "community" aspect of the site.
On a "business plan" side of things, a nice round figure like 20-25 $/year would be quite acceptable and easy to manage (paypal, credit card etc). IMHO, just a look at the "Friends of FAOL" can give you an indication of the "readership" which is ready to pay for quality service.
My 2?