Before I worked in aerospace as a structural engineer, I couldn't understand why airplanes are as expensive as they are.
Once I'd worked in aerospace as a structural engineer, I couldn't understand why airplanes aren't more expensive than they are.
I worked in stress analysis, where you'd analyze a part of an airplane to determine whether it fulfilled strength/fatigue life requirements. At this stage, the part's already been designed. So you take the part's material properties, geometry, surface preparation and coatings, machined features, and so on, and check it against anywhere from a few dozen to many thousands of load scenarios. You find the handful of critical load cases, and then you write up the analysis against those. The analysis is intended to be mostly self-contained, readable by any qualified engineer, and serve as a complete treatment of the part's structural sufficiency. It's not much of an exaggeration to say that every single structural component of an airplane is backed up by a stress analysis report running somewhere between 20 and 400 pages, written by one or more degreed engineers making what was, in the U.S. probably an average of $50-60/hr. Airliners are made up of many, many structural parts. The costs of proving the safety of those parts add up quickly.
Now, if I had to guess, instruction manuals are probably a bit cheaper to produce. Tech writers probably aren't as well compensated as engineers. But a lot of engineers' time will be eaten up nonetheless. (I once spent a memorable few weeks (!) proving that a dowel pin on a part that hadn't been machined correctly would not, during installation, shear off, so that existing installation documentation could still be used. Yay. After that I was known as the office dowel pin SME.) Good maintenance documents sweat the details and require input from designers, stress analysts, manufacturing engineers, supply chain representatives, and others. And then as the design evolves -- the design always evolves on the way to delivering a product -- these documents must be tweaked, added to, subtracted from, sometimes entirely rewritten, and those changes must be checked by a variety of folks.
So, $84 million for 100,000 pages might sound like a lot. But for what is in many ways a custom built aircraft, with divergences from the base aircraft that probably run into the millions, to me, it doesn't seem ridiculous.