The above may seem harsh but thats the way it is.. client needs a job doing, goes to agency A and says I can pay X per day/hour. The agency will do almost everything to keep as much of X as possible. Your contract is with the agency NOT the client, so its up to you to negotiate how much of X is up for grabs - be ruthless, be prepared to walk away and you'll benefit in the long run.
I've gotten agents to go as low as 5% and yet been stuck at 20% for a whole year. Its all about your skills, length of contract and current state of the market. It also helps to assume that ALL agents are several rungs below car sales people when it comes to honesty.