Typical expenses for publishers to publish one paper open access are in the range $1000-$3000 for your average journal. Source
ht @PaulGuinnessy
physicstoday.scitation.org/doi/abs/10.106…
ht @PaulGuinnessy
publications.parliament.uk/pa/cm200304/cm…
nature.com/news/open-acce…
The bottom-line is, while exact numbers differ from journal to journal, from field to field, it takes publishers about $1000 at least to publish a paper. That money must come from somewhere.