About $340 for the full set of eBooks. Also: "Please do not tell me about errors that you find in an eBook, whether it's PDF or not, unless the same errors are present in a printed copy; such mistakes should be reported directly to the publisher." Glad he thought to mention this, but I suspect his inbox will still be inundated.
He doesn't have an inbox: https://www-cs-faculty.stanford.edu/~knuth/email.html
E-mails which are sent in about the various books used to be printed out and responded to --- I got a $2.88 physical reward check for finding an error and a point of improvement in _Digital Typography_. Not sure how they are handled now. Trying to find an error or point of improvement in v4f7 so that I can get an account at:
https://www-cs-faculty.stanford.edu/~knuth/boss.html
(usually, I do find errors in books, esp. e-books, which reminds me, I need to pick up the corrected 3rd printing of _The Fall of Arthur_ by J.R.R. Tolkien before I read it again, since that should have the error I found corrected).