Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
How did the printing press take over Europe in fifty years? (for a curious adult who has not studied history). Cover, in this order: How were books made before the printing press?; What exactly did Gutenberg invent?; Wasn't printing invented earlier in China and Korea?; Why did printing jump from city to city instead of spreading like a ripple?; Why did books become so much cheaper?; What changed once many people could read the same words? — page 5: Why Books Got Cheap
By hand, copy number two costs almost as much as copy number one. Every single page is written out again, letter by letter.
Printing works differently. You set the type once, and that same setup stamps out hundreds of identical pages.
Pressing a sheet is much faster than writing it out by hand. Afterward the metal letters are taken apart and reused for the next page.
Within decades, prices fell sharply, so students, merchants, and townspeople could finally afford books. Ownership by ordinary families was still further off.
But paper did not get that discount. Every copy still needs its own sheets, so materials stayed a big part of the price.
Print made the effort of making a book a one-time cost instead of a cost paid again with every copy, and that is why books reached far more people.