I'm not sure about other cities, but here in the Pa. area most of the bigger franchise gyms like Bally's and L.A. Fitness make you sell your ass off in P.T. sessions for your portion of the money. Plus, they pay you a % of what you sold. So if you sell the client on the highest rate per hour they have at the gym you make more. If the client goes low ball and can't put out that much moneny per session you get much less. To me it's not worth it if you're looking at it from a $$$ stand point. BUT, if you're looking at it from a experience stand point then it's great. Because all your equipment is right there for you to use. And all your clients are right there also. This gives you tons of great experience working with so many different types of people.
If you're gonna do it, go for it and get your experience then get out and make better money! Good luck.