How do medical billers charge Solo physician practices?

I'm contemplating on billing and submitting claims for a family practitioner for his private practice.

Do medical billers charge per patient? How much do they often charge a physician for billing and submitting claims per patient?

Do medical billers charge for the amount of patients that were seen in 2 weeks or do they charge for the amount of patients seen in a month?

Do medical billers often charge a particular percentage of the total patients that were seen by a physician in a month, 2 weeks etc.? If that's the case, what percentage do they usually charge?

How do medical billers charge Solo physician practices?