What causes the cost of an eye exam to vary?

Market: A variety of things, such as the area cost of living, insurance/ medicare rates, the doctors rates, area competition, advertisement costs, "reputation of doctor" factors, etc. Insurance/medicare set allowable fees covered and co-pays, the doctor may set his/her fee equal to, or above allowable and the elect to either write off the over charges or require patient to pay. Some fees are pay only.

MD or OD: Ophthalmologists are mds. Optometrists are optometry doctors. Usually the fees for mds are higher, although insurances reimburse the same. An ophthalmologist goes to medical school 8 years and then 1+3 years of residency before specialization. Optometrist go to school 6 years after high school. Varying cost may include some doctors that dilate the pupil the first exam and thus the cost is more.