You seem to be heavily focussed on Github's financial costs, but why should I use this over something like Bitbucket, which provides free hosting for small teams and very reasonable pricing for teams?
In a financial standpoint, once you hit over 5 users on BitBucket, you're paying $1/user/month. With Legit Teams, you only pay for space and are not limited on either collaborators or repositories. And for me personally, the 500 MB plan is more space than I'll ever need, as only the size of my compressed .git folders count.
As the owner of a small non-profit with real turnover, I would much rather use something like this.. just my two cents. We've had issues with holdover accounts costing us even after our developers are no longer actively involved. It makes a lot more sense to charge based on storage space.