I'm not sure I agree that they have a right to exist, hence to have a business model. A good inspiration (on a smaller scale) can be the ACL and the recent open

I'm not sure I agree that they have a right to exist, hence to have a business model. A good inspiration (on a smaller scale) can be the ACL and the recent open access journals (JMLR, JAIR, etc), that are mainly university-sponsored or university-consorcium-sponsored, have very low operation costs (not having to send all that spam and publish empty magazines like communications was for a long time certainly helps), and volunteer-run (which is not a problem in academia if you attach some prestige to running things). Maybe a subscription model that gives discounts to events for those who pay for the annual membership, with something like discounted book deals and informational emails sent periodically.