I think the question is much like any other person who enables client's to operate in any business.
Having escaped from the music industry I have had more than enough of middle men and chancers.
The same question come's up with Agent's who represent you when you are looking for work or solicitor's who under line your effort's and make you all legal.
A really good one who is trying to help you build a business and has an eye on building a future will start out charging you any thing form 0% to 15% of your money,they will encourage you and help facilitate growth before trying to make any real money out of the situation.
Then you have the established agent who has a lot of clout and will know how to elevate your work to a higher earning potential through knowledge and contact's,they will expect any thing from 20-30% of your money but are worth every penny if you are taken under there wing.
Finally you have the the Greedy operator,They will promise you any thing,Make deal's then weasel there end up any way they can.Be prepared for tantrum's and name calling and generally the attitude of a spoiled little girl.they will start out by demanding half your money and will intend to take you for as much as they can.
From my experience that is basically the three type's of operator in most businesses operation's. obviously there are many subtypes but you get the Idea.