It's not like this is some document that's going to be downloaded a billion times, so whether it's distributed via bittorrent or some government document site isn't going to matter.
That is, the cost comes from somebody having to dig up the original document in paper or microfilm, scan it, and then put it up on the document site. And then all the kinds of overhead costs associated with having an physical and electronic archive like that (facilities, salaries etc.).
If everybody on the planet suddenly develops an acute interest in WWII era cowl flap indicators, sure, the above costs amortized over billions of downloads would be very close to zero. But presumably the government guesses that this document might be downloaded maybe a hundred times over the following decades and prices accordingly.
