People that manage producers find work for the producer. That is reflective of your skills. If you can mix, he can find you a mixing job, if you can master, he will find you a mastering project. If he can broker your beats for you, you... and he... will get paid.
The whole purpose is for him to find you work, and he receives a percentage of the money you get per job. Or, he can broker deals for you with agents, etc., which then gives him a percentage cut. Basically, he finds you some form of work, or money for your work, and you pay him the commission.
Remember, a lot of con-artists will do something like this: They (your 'manager') tells you that he got a quote for 5.000 dollars. You don't know the source of the job, like what artist, or what executive producer, because you trust the manager. In fact, the job may be for 10.000 dollars, for example, so the manager will pocket extra 5.000 dollars AND get commission off your job. It may not be that extreme, where half the money is pocketed, perhaps only a 300 to 500 dollar skim off the top, either way, it happens, AND IT IS WRONG. Protect yourself.
That is a simple scandal to be aware of.
Regarding percentages, they are usually negotiable.
Sincerely,
God
THIS POST DOES NOT CONSTITUTE LEGAL OR BUSINESS ADVICE. IF YOU DO NOT KNOW SOMETHING, SEEK THE COUNSEL OF A QUALIFIED ATTORNEY.