It’s not just the printing cost but storage, transportation, distribution and collection (though free books get collected; most have the covers removed and the paper thrown out). Plus there is the setup for commercial printing and minimum runs. The 100,000th book it may only be a couple dollars, but a run of specialty texts that only has a 10,000 print run could be ten times that. Shipping cost, alone, of three pounds of dead tree can cost in excess of $12-15 by ground transport (at retail rates) plus packaging and handling.

It was once said (in the aerospace industry) that every hand which touches a part costs $100. While it’s less for items like books, the number of transfers from publishes pc to a students backpack is pretty large.