What determines how much an employee earns?

According to Freakonomics

The four meaningful factors that determine low wage [WUDS]

-No specialised skills required
-Many willing and able to do the job
-Not so unpleasant
-Low demand for services that the job produces

The delicate balance between these factors helps explain why, for instance, the typical prostitute earns more than the typical architect.

SKILL LEVEL:
It may not seem as though she should. The architect would appear to be more skilled (as the word is usually defined) and better educated (again, as usually defined).

WILLINGNESS:
But little girls don’t grow up dreaming of becoming prostitutes, so the supply of potential prostitutes is relatively small.

UNPLEASANT:
Their skills, while not necessarily “specialized,” are practiced in a very specialized context. The job is unpleasant and forbidding in at least two significant ways: the likelihood of violence and the lost opportunity of having a stable family life.

DEMAND:
Let’s just say that an architect is more likely to hire a prostitute than vice versa.