10% would be on the low side for either a recruitment agency or, to run with the Ari Gold analogy, a talent agent. 15% would be more realistic, although I see recruiters trying to charge considerably more. Unscrupulous recruiters often will try to hide how much their cut is from their freelancers.
Hollywood agents are capped at 10%, Hollywood managers typically are more like 15%. Agents are "forbidden" by California law to produce projects, though that line can be awfully blurry. Managers can't, in theory, directly negotiate contracts. In theory.
A lot of talent ends up with both a manager and an agent, each with their own percentage.
Toss in an attorney at 5+% and you can see how the Hollywood money really flows.
As more big firms opt for management comapnies such as ResourceSolutions, this is been driven firmly down. Often its below 3% for people who've been on site 18+ months.
The worst thing about RS (guess which side I'm on!) is that they advise on rate reductions. As they are often working with your competitors too, they can effectively advise that a 10% hair cut is OK because OtherBank just did a n% cut.