Ken and other authors of ebooks can't be held responsible for the over the top prices charged for ink cartridges (far more expensive, I read, than French Perfume).
Just comparing the cost of a printed textbook with an ebook is a bit misleading.
Because I am more comfortable with hard copy, I usually print out the ebooks I buy and refer to both versions as needed - the onscreen one to find something which is not indexed and the hardcopy for more comfortable reading.
But I print most of them in black only, 2 pages per side and double sided which reduces cost (on something like MYSS2000, that's quite a bit) though it takes more time and I have to reprint some charts, etc full size.
And I only print those sections which I need to.
And Ken's books, like Ricky Higgs and others have proved well worth the fairly low investment in $$'s and time to print them out.