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.

I help out with the production of a periodical that is journal-ish [0], and the biggest expense is printing and mailing. But it's ran by a non-profit, our editors are all volunteers, we don't do peer review, and our authors typeset the articles themselves, so this is definitely an atypical example.

[0]: https://tug.org/TUGboat/

This is a silly question to ask. What do you expect a rent seeker to say? Of course there are costs. Real estate brokers have costs, Apple store has costs, a publisher has costs. That's what they'll say. It does not matter what the costs are. The fees are what the market bears.