I don’t think Mother Agent should/will charge you 20% that much

if your current agent is not = your mother agent. You might be confused Mother Agent with your working agent.
Mother agent is your 1st agent who training you to be a professional model and they will always get % of money from your assignment for the rest of your life. And they will help you to negotiate the price, or any kind of help when you need. If your Mother Agent is powerful, such as FORD, then you are in good shape.