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.
So I do headhunting for both sides. I get paid a commission from the company for finding them great talent(I use already established connections of mine so I already know if they are a good fit for the job or not), and the person that gets hired pays me a commission for finding them a job.
It's typically 20% of their monthly salary for 6 months from both sides.
Good deal for a person to make some extra cash on the side of whatever they do already, and they have time to do so.
If you look at it from the perspective of the employee, it's quite possibly their biggest single expense all year. 20-30% of salary is common. It's easy to see why recruiters are paranoid about getting stiffed by the hiring company.
Unfortunately, it also puts you at a disadvantage when negotiating with the hiring company. The next guy who applied for the same job but didn't have to go through the recruiter would cost the company the same even if they earned 20% larger checks.
Still, as bad as recruiters are, the companies themselves are worse, by and large.
In order to be successful you will probably require me not to leave the new job for at least 6-12 months. That can be an expensive term to have because of the risk being stuck in some crappy workplace, the risk of missing better opportunities and some other more subtle limitations that costs me money. That can drive your cut down significantly. On the other hand if you have premium companies like Google, you can claim a bigger cut and it's still a good deal.