Any way you look at it, if a client paid someone $750, and that person turns around and pays someone else $250 to do the job, and that DJ actually facilitating the job brings all their own equipment, handles every aspect of the job outside of booking the job, and providing an agreement, and collecting the payments, then the the person who handled the booking is simply a shyster taking advantage of someone else. Bringing in a DJ for $250 because the person is new or desperate for money/work but charging the client $750 is what snake oil sales/business people do. They are taking advantage of the DJ.
Also, I KNOW 100% that a business person/company that operates in this fashion was not on the up and up with the client. NEVER EVER would they say to the client "Hey listen, I'm going to charge you $750 for a DJ, and then hand off the paper work to your DJ. I will pay them $250, and they will be the person you communicate with from here on out. Next time you hear from me will be after the event to see how everything goes. Is that cool with you? "
NEVER EVER would this person be honest with the client.
So to answer your question. Yes, they are taking advantage of you. They are exploiting you, the laborer. The one doing all the work.
Now, if the company/booking person was providing all the equipment to facilitate the job, and handled all the planning aspects with the client...then perhaps I can see an arrangement like this being beneficial to both parties...maybe 45/55 or 50/50...not 33/67 unless the Van/Car, and fuel money, and company polo shirt was provided too.
