In the light of recent publications, it doesn't hurt to go over a few simple calculations yet again:
"even when considering all services we don’t currently pay for, the true cost per paper would not exceed $100"
https://blog.joss.theoj.org/2019/06/cost-models-for-running-an-online-open-journal
This fits very well with what we have calculated:
https://f1000research.com/articles/10-20/v2
and of course many others as well (references in our paper). Remember, if someone tells you academic publishing costs more, they're not doing it right