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.
CEO of EMS Press here (publisher of the European Mathematical Society). Like most society publishers, we really care about our discipline(s) and want to support researchers regardless of whether they or their institution can afford an astronomical APC or subscription rates.
Good publishing costs money but there are alternatives to the established models. Since 2021 we use the Subscribe to Open (S2O) model where libraries subscribe to journals and at the beginning of each subscription year we check for each journal whether the collected revenues cover our projected costs: if they do we publish that year's content Open Access, otherwise only subscribers have access. So no fees for authors and if libraries put their money where their mouth is then also full OA and thus no barriers to reading. All journals full OA since 2024. Easy.
Less boilerplate than the other solutions I know... for example it allows you to define types easily just by writing a parser/validator ("transformation" in fefe terminology).
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.