That's something worker unions can help with, by enlightening members what a reasonable salary is. These consultancy companies often underpay their consultants. Sometimes the salary is egregiously low with the excuse that they are providing some kind of preparatory education for some length of time. The work of that same consultant that is "in education" is then heavily overcharged for.

I've heard some horror stories with these.

tl;dr Make use of a good union or org to find out what a good salary is and don't get yourself ripped off.