It sounds like it would be a big help if github offered unlimited read-only/bot accounts. Not sure how technically feasible is that but it doesn't sound impossible.
Or as someone mentioned above, if they went the Slack route of not charging for users who don't push/pull code in a 30/60 day period. That way you could still have collaborator users (issues, PRs, etc) and only pay for the users who actually code.
