How do locums companies work?

I'm specifically wondering how they structure their profit? How much money do they actually make from us and is it a cut of our hourly rate? As in for every hour we work, they make a certain %? and what is that actual %? I am wondering since I have been kind of shocked at the low ball locums offers I have seen and in talking to recruiters I get the sense that they profit off of us big time.
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums