It depends. I've done it several different ways - sometimes by the chart note which can be between $20-$40. (Normally, per chart is more $$$)
Remember your time is valuable, your an expert in your field - take that into account always! As well as how much time is involved, and travel if any.
You can charge flat rates i.e. $xxxx for first provider, each additional provider $xxx
or even by the hour... if by the hour $100/hr is competitive... if not too low! I've gone as high as $200/hr and as little as $100/hr.
There's so much that can stem from auditing! it's unreal...... not just "coding accuracy" but what about the financial impact it has if they are severly overcoding or undercoding? what happens if you find a problem, i.e. insurance isn't paying according to their contract for the last 5 years. ANYTHING can happen - remember that. Keep your options open, and be available and ready because they'll ask you for additional help and services.