> If it becomes at all popular then you have further ongoing overhead.
THIS is the one that comes into play. All that stuff you listed before is a slog to go through when you go to open-source, but unless you're parking the thing as a proof-of-concept/end-of-life, you're now signed up for maintaining the repo. That means triaging issues, pull requests, helping out when contributors don't understand why their build is breaking, etc.
And there's the PM aspect of it: you don't just want to develop a bunch of features without talking to your community, so communicating (in a public friendly way) what you're planning to work on, when folks might reasonably expect that, how they might be able to help out, which of their contributed features you might be able to take depending on where you're at in your lifecycle, and RESPONDING TO COMMENTS all takes way more time than just "building [closed-source] product" in a team of 5-20.
And of course, one of the hardest of all: telling people "no, we can't take that change" when they've spent hours and hours doing work for your project for free. In that regard, we're still very much iterating on a transparent design process that allows for consensus BEFORE too much work has been done (though as we all know, building prototypes is often one of the best ways to find out if a design works right or not).
If you're doing it all right, everyone involved in the project should be doing some amount of all of this every single day. There's no compartmentalizing an engineer on an OSS project as "someone who just writes feature code" vs. "someone who does the repo stuff".
So going back to OP's point: no, it doesn't literally "cost anything" (or very much) to do the basic act of open-sourcing, doing it the "right way" at scale where you're truly engaging and working with the bazaar is very expensive.
Full disclosure: I'm a PM at Microsoft working on PowerShell and was heavily involved in it being open-sourced and ported to macOS/Linux.