Since lurkers/readers don't generally use storage space, they can contribute during the Quarterly fundraisers.
However, for those with accounts who post, myself included, there should be a fee per post. Maybe $.10/post or .25/post. You'd have a better idea of the cost. On a quarterly basis, either a credit card or PayPal could be used to collect the amounts due.
This would be self-regulating. Someone with bad Karma about Free Republic would be supporting it every time they posted a vanity/whining post. Those who over use it and post asinine stuff would be supporting it. Actual articles posted should be either free, or x number for .25, since content is critically important to keep the site relevant. Replies are what should be priced.
With this system, those who are disruptors, would also pay to run Free Republic. What could be better? Those who are already sending money, could continue, or modify their payments. This pay as you go, is the only way to avoid the constant drain on your time and resources, and will grow with the site.