I used to use copy reviewers and editors quite a bit. We usually worked with market specialized editors, e.g. IT, legal, marcom. For us it was important that they understood the context and quite often specific (technical) language.
I recon your competition can be found with the many online services. Nowadays you can post your document with one of the many online services, fill in your requirements and you usually have feedback within 24 hours.
Rates vary from fixed price to 0.01 - 0.15 cents per word depending on speciality/required skillset.
The standard still seems to be charged per word so that's what I would use. Perhaps 0.05 cents for standard work and 0.10 for specialized work (e.g. legal documents).
Hope this helps.