How much does a publisher charge authors if they want to make their book open access? Is there any research/study/survey that looked at how much publishers charge authors if they want to make their books (textbooks or research books) open access?

I am most interested in the field of computer science, and English-speaking venues, but interested in other field and languages as well.
For SpringerOpen to publish an open access book, author fees vary according to page length, where 525 words = 1 page, and costs start about €7,000.

One option to publish a book for free would be to self-publish it using the open-source, free Open Monograph Press software.