Yeah, I don't think technical authors (of the sort that used to write for the paper magazines like EDN, EET, and so on) ever got paid "engineer wages."
Then again, a lot of them (plus the more scholarly journals) leveraged off of engineers who were already getting paid for that time, possibly plus a "publication bonus", presumably for the good publicity for whomever they were actually working for.
There's probably a reasonable number somewhere, but both 0 and $100/hr aren't it...