You're obligated to pay them whatever you sign to pay them. Normally, a manager is going to charge 10-15% of your check. The only weird thing about managers vs. agents is that they're not supposed to actually secure you work, i.e. send you out on auditions. That's what an agent is supposed to do. Nevertheless, every manager in the world still does it. The "rules," which seem to be enforced by nobody, are very fuzzy. Is the manager sending you out, or did he hook you up with an agent?
Never heard of ol' Jamie, though. But that doesn't mean much, because I don't know shit. If you go to Drama Books, you can drop $15 or so for a Henderson's guide to managers in the area. Could give you a tiny bit of insight.