So perhaps taking the extra couple minutes to write the response may come out about the same. All the students get the answer, and can reread it at their leisure. We might come out ahead in asynchronous environments, depending on the instructor and student.
Costs are equally evasive to measure.
Suppose, for argument's sake, that they travel an average of 20 miles in each direction. Then, for comparison purposes, we would have to look at average computer usage and maintenance costs, server fees, and a percentage of telephone costs. I think it safe to say that less physical energy is expended in the online environment. You also have to look at the probability of client dissatisfaction, and the cost of errors and omissions insurance. In my experience, online teaching produces lower gross revenues at considerably lower costs and risks, so I actually net out more teaching than with the typical client contracting for the same gross fee.