While I am willing to pay $20 a year, hell I would pay $50 a year for this site, it is not a good solution. What got me to the site at first place is the wealth of information available and accessible. It would be difficult to get new users/members to join if payment is required before hand.
Banner ads could work, or why not have a yearly donation drive ala Wikipedia? $5, $10, $20 from decent number of member should cover cost.