For many situations, a medical expert is just fine for this job. But in some scenarios, it can be more valuable and efficient;
1. Your medical expert is an expert in his/her field of medicine; not in the billing department. Their administrative staff tend to know more about their billing than they do.
2. Although your expert may be willing to testify to his/her own charges being reasonable and customary, they might not be qualified to testify regarding another practitioner or modality.
3. Instead of having declarations, testimonies, etc. for each medical facility and each medical provider, you can combine them all into one job; this will save a lot of money on expert fees and staff time.