I don't follow, was $9/user/mo (presumably what the Team plan was) low enough for it to be "effectively free" and invite problem customers?

I would have thought those sorts of people just go for the $0/mo option here.

Presumably.

It's also possible that some organizations incur more than $9 per month in admnistrative or other costs to bill a single customer.

Maybe if you're billing with a paper and pencil, mailing out invoices, and you have to drive a bag of checks over to the bank.

BuildKite is and orchestration SaaS, they don't even own or run the hardware the agents are on. If it costs them $9/mo/user to manage that most of the C-suite should be fired.

Do they use another $9/per month/per customer SAAS subscription to achieve that?

You'd be surprised how many firms have subscriptions to billing insights, know-your-customer, fraud detection and other similar services. Many of the "AI" companies that are emerging do such things.

That wouldn't surprise me. What would suprise me, though, if any of this (individually or in aggregate) is such a huge expense on a per-customer basis

I see the edit but I think you might not be too far off. I get the impression that businesses whose customers are engineers can be some of the most difficult to satisfy. Perhaps its easier to focus on the larger customers going forward.

Engineers as customers are either great ("hey, found a bug, <detailed report with suggested fix>") or terrible ("hey, found a bug, <long rant about how they could have written this themselves and how terrible your developers are and how they are going to leave for competitor X and ...>").