With the minor exception of twins doing the exact same manual labor, the very idea of "comparable work" is impossible. Each individual brings a unique basket of talents and abilities to every unique job. It is impossible for some outside agency or court to determine some kind of comparable work standard in this situation.
The only fair assessment is the market. People agree to work for whatever they agree to work for. If they accept it, it is fair. If they don't, then they go elsewhere.