I do not find that they do. A large part of the cost is generally from the parts themselves rather than the labor and overhead, which is generally equal to about half the unit rate (the other half is for the actual labor, what the mechanic earns). Overhead includes the upkeep of the facility, the costs of operations including (especially) business and liability insurance, and pay for support staff. The mark-up on parts is usually quite reasonable as well so they do not generally screw the clients. There are some who do, but many do not.
All services face the same basic economic concerns: labor costs, overhead costs, and whatever they can get above that for profit. You will probably find out at some point, if you ever end up in a management position at any small firm (or even larger firm).