Fees depend on the respective mohel and vary according to several factors including whether or not the mohel is also a physician and therefore held to a higher standard than a "non-physician" mohel, their professional status of full-time versus part-time, distance traveled to the ceremony and finally the demand for a particular individual based on their experience and popularity.
Specifically, a physician/mohel is held to a higher legal standard than a non-physician mohel. A physician/mohel is required to carry medical malpractice insurance. Though expensive, liability insurance of this kind protects both the physician/mohel and the family. The physician/mohel is also required to be licensed to practice medicine within the state in which the Bris is performed. This licensing cost too is absorbed by the physician. These same expenses are not borne by "non-physician" mohel. The "non-physician" mohelim are not licensed by the state nor insured.
If a physician/mohel practices as a full-time physician and maintains an office, their work as a mohel becomes secondary. The situation with a full-time mohel is different of course. The fee for a full-time mohel will be higher as they depend on it for their living and to reimburse expenses normally covered by an office-based practice of medicine.
The distance to a ceremony can be a significant factor. Often families tell a mohel that they live in the neighborhood, but the ceremony will be conducted at the grandparent's home 75 miles away. So, the fees will always be predicated on a consideration of the distance to be traveled
Finally, as opposed to a commonly held belief, not all mohalim/ot have equal experience and capabilities. A well regarded, experienced and popular mohel, both physician or non-physician will generally command a slightly higher fee.