Early in my career I worked for a consulting firm as an employee, and I never found out exactly what I was billed at but I'm guessing it was around double what I was paid. We worked on site at our client's office, and people there called us "contractors," but we were employees of the consuting firm. We had benefits, training, overtime pay, etc. and we got paid our salary between jobs when we were not on a client project.

If I were an indepenedent contractor using an agency to find work, I'd expect them to take a much lower cut, as they have much lower overhead and risk.