Anytime I'm choosing fees related to external review, I consider a number of options:
1.) The source: A single-physician practice is probably less willing to pay me what a law firm might.
2.) Deadlines: If I have X amount of days/weeks/hours to review, that is going to dictate the charge I request. My time is valuable.
3.) Type of review: If I'm reviewing an inpatient, two-month hospital stay for Pro fee and DRG, etc, I'm obviously going to charge more per chart than I would for 10 Family Practice encounters.
4.) How much work is involved for you? I don't like flat rates and I don't like the cookie-cutter idea (for myself, at least) of charging the same thing to every client. Each one may have specific targets, budgets and involvement in the process. I try my best to be fair to both parties (myself and them) and really think critically, rather than with my pocketbook.
Just a few thoughts and suggestions. Nothing specific, I'm aware, but hopefully it'll help you create a framework for your own decision.
Good luck.