Students at the university had to use their books as collateral, it seems, and if they didn't pay off their debt before they left, the books would have to stay with the university.
Professor DeLong goes on to mention:
Back in the thirteenth century, a book copy sold for the same share of national product that $14,000 is today.
Just prior to that:
At the end of the fifteenth century, a book copy sold for the same share of national productivity that $1400 is today--the equivalent relative expense to this laptop I am typing on.
The ten-fold drop in price is largely due, of course, to Gutenberg.
I always thought that putting fantasy-era RPG prices in dollar equivalents, or at least having dollar equivalents floating around, would do a great job putting things in perspective.