Market rate = Zero. In practice recruitment consultants will be taking 20% or more of first year salary, but that's from the employer and is therefore not net of income tax, and possibly not net of salary brackets, for employers that have them. More importantly the job seekers are able to take the recruiters' advice (for what its worth, which may be nothing) and go and find their own job through their own efforts and not owe anything.

There are companies and individuals that make a living from commissions paid by employees for getting the job, and there are probably individuals that would benefit from getting paid career consultancy, but when the competition is free at the point of sale it's not an easy market to get into.