Re: How can it possibly be worth that much?
"they do charge money for private repositories"
and it seems to be cost-effective for small development projects in which you have one or more people working from home (or even more remotely). it's actually pretty good that way, as you can use issues and pull requests to manage collaboration, and the wiki to manage documentation.
I have a previous customer that managed their project this way. For the most part, it worked well. When done right, you could test a change in a branch, then commit it, and do 'git pull' and re-build things [as needed] in a very short period time, to update the production system. Down, git pull, build, up. pretty fast. And private repos, so the 'secret sauce' was kept secret [still had open source but it was in the form of taballs that were snapshots of firmware releases, not day-to-day history visible to the public].