I imagine most of the cost in the OP isnt the physical writing of the documentation, but rather in the time gathering the documentation, interviewing the engine

I imagine most of the cost in the OP isnt the physical writing of the documentation, but rather in the time gathering the documentation, interviewing the engineers and developers. Then on top of that, might need to employ separate engineers to review designs/code and verify that the information is correct and accurate. Effectively a design/implementation audit. If done truly independently, I can imagine this gets quite expensive very quickly. I'm sure the number of lines of code involved in a 787 is probably well into the 10s of millions. Then you've also need help of Electrical/Computer engineers, mechanical engineers, aerospace engineers and who knows what else. This isn't a simple matter of plopping a tech writer in front of a computer and "punch out the doc". I dont imagine the systems are simple enough that you have a single engineer or developer looking at the relevant systems. I can easily see a team of well more than 50 being needed to produce such documentation, and conclude the cost of production for such documentation being $1M USD to probably being way on the low end, maybe even by an order of magnitude or more.