Happy to share details! Typesetting is a big item (for us becoming even more due to production of accessible publications), language editing, (meta-)data curation, technical infrastructure and software development (peer review systems, hosting, metadata and fulltext deposits, long-term preservation, maintenance, plagiarism and fraud detection), editor training/onboarding, editorial support, marketing, and of course our staff running all of this also wants a salary.
Some keep repeating that Diamond OA is superior because publishing is free for authors and everything is immediately OA. And indeed it is, but only if you have someone who is indefinitely throwing money at the journal. If that's not the case then someone else pays, for example universities who pay their staff who decide to dedicate their work time to the journal. Or it's just unpaid labour so someone pays with their time. It's leading to the same sustainability issues that many Open Source projects run into.
> Typesetting is a big item
I'm calling bullshit. Look at how annoying the template requirements are for authors: https://www.acm.org/publications/authors/submissions, and note the stuff around Word files. Other journals can be much worse.
If any serious typesetting were being done by these journals, simple plaintext, Markdown (or RMarkdown) or minimal basic LaTeX, with, admittedly, figures generated to spec, would be more than enough for typesetters to manage. In fact, if you were doing serious typesetting, you wouldn't want your users doing a bunch of formatting and layout themselves, and would demand more minimal representations of the content only. Instead you have these ridiculous templates. I am not convinced AT ALL.
Do authors submitting to literary agents have to follow such absurd rules? I think not. Can modern blogging tools create beautiful sites with simple Markdown and images? Yes. So why do academic publishers demand so much from authors? IMO because they are barely doing anything at all re: typesetting and formatting and the like.
Thank you for contributing your expertise and experience.
> long-term preservation
How is that done beyond using PDF/A? I'm interested for my own files.
> Typesetting is a big item (for us becoming even more due to production of accessible publications), language editing, (meta-)data curation
I'm sure you've considered this idea; how does it work out in reality?: What happens if you push one or more of those items onto the authors - e.g., 'we won't publish your submission without proper typesetting, etc.'? Or is that just not realistic for many/most authors?