The total cost for x-ray film for the day's services does vary with volume. Other costs can be classified as direct, indirect and intangible. Direct costs are those that occur when providing the service. Indirect costs do not result from the service. An example of indirect cost would be the loss of time from class work.
Intangible costs are factors like the quality of life.
Economies of scale assume that a greater volume of a given service results in lower charges. Fixed costs don't change with volume. The greater volume results in the fixed costs being spread out over a greater number of services. A smaller portion of the fixed cost maybe allocated to the service thus resulting in a lower charge. Sometimes it is not possible to offer the lowest charge.