I would suggest for a flat fee depending on the no. of hours you would need to spend on this assessment project. It would be more like an expert analysis or consulting, where you will be required to review
1. the types of services they bill and their volumes
2. their reimbursements pattern (par vs. non-par rates)
3. their cost of billing including manpower, billing software, Clearinghouse charges, Infrastructure, etc.
4. their follow-up process on unpaid claims and statements and their share with collection agencies (if applicable).
5. their credentialing and prior authorization processes to fix any gaps there.
So, it purely depends on how well and how soon you will be able to perform these tasks. You may possibly mention your location, so that we can expect some expert advise from others who know about the salary in that location. Good luck.
Regards
Sriram