I'm an experienced editor, writer and layout person who has been working of late for tech, IT, financial and similar companies doing copy writing, editing and proofreading.
In my misspent youth I was much more adventurous and worked in places such as Japan, Hong Kong, Singapore, India, Pakistan, Nepal, Kuwait, Thailand, Beijing and Somalia. A lot of my work was as a stringer, some of it was doing research and background for the actual writer, in some cases I was the guy holding the camera or lugging equipment while dodging bullets and shrapnel. I believe that was called building character in the old days.
Considering all of the above, I'd like to propose the following;
1) Starting at a per page rate of US$2 for basic editing (checking for grammar, spelling and continuity in the documents)
2)An hourly rate of US$15 for researching background (regional/ethnic slang, military, police or occupational jargon and procedures) for your authors.