Job Requirement

4 factors determining whether a not a job is high-paying: (rephrased from Freakonomics)

1) Abundance (supply) of able doers.
A job will not be high-paying if many are able and willing to do it, i.e. janitor, receptionist, etc.

2) Specialization
Rare and highly specialized skill is a valuable resource. Such scarce ability is worth more than other skills that are more available everywhere. (Example: neural surgeon vs. general practice)

3) Unpleasantness nature of the job
If a job is unpleasant, less people are willing to do it and thus a higher incentive has to be offered to attract doers.

4) Demand for services that the job fulfills.
If person A has a set of skill that can match the demand easier than person B of a different set of skills, person A is more likely to land a job than B, no matter how specialized B is.

So a prostitute may easily earn more than an engineer. Even though the engineer is more skilled and better educated (as usually defined), little girls don't dream to become a prostitute so supply of potential prostitutes is relatively small. The skills, although not exactly specializing, are applied in a very specialized context. It is very unpleasant due to the tendency of violence and the lost opportunity of having stable family. What about demand? Let's say an engineer is likely to hire a prostitute than vice versa.

Bear in mind that the inner working of the above was discussed within economy's context that is amoral. Therein lies the problem, isn't it?