The time it takes a physician to make a diagnosis and prescribe a remedy would not be possible without many years of studying medicine, his/her continuing studies to keep abreast of latest medical discoveries, and his/her expense of keeping an adequately equipped and staffed place of business. You pay for his knowledge and skill plus a share of his cost of doing business, not for the few minutes he spends with you.
Similarly, the charge in for providing competent service in your home cannot be determined solely by the time it takes the technician to diagnose your appliance and make repairs.
A qualified service organization's costs begin with the salary of the professional technician who provides the service. This amount is the same whether the technician is actually making repairs in customers' homes, traveling to keep appointments, or participating in training to sharpen his skills. A technician typically spends less than half his time in customers' homes, his company's cost for each hour in your home is considerably greater then the technician's hourly wage.
The cost to a service organization (except for the technician's time on the job) could be about the same for a simple job of replacing a fuse as it is for a difficult job.
When you are pleased with the service you are given but still question the fairness of the bill - even after taking into account these somewhat less visible, but very real, elements of cost...question it! Call and tell the company that provided the service, and ask to speak to a manager. Usually that's all that's needed to get a satisfactory explanation or an adjustment if one is justified.
When that professional service technician knocks on your door, many costs have been incurred just to get him there, ready to do your job.