All about how you sell yourself really. For most PT's there is not much money to be had, but once again it all depends on how well you can sell yourself and what type of clientele you can sweep in.
My good friend charges $90/hour doing PT, but then again he's the owner of a very popular gym here and has a big reputation. On the flip side, the other guys who do personal training at that gym don't make even close to what he makes, especially after you factor in the fee you gotta pay the gym (since you are using their premises).
You'll have to pay Bally's a fee, so use them to procure some clients and making contacts. Although this may be wishful thinking and easier said then done, every city has there high class gyms in which you can rake in the dough. For instance, there is a gym here that I sampled a few times. A lot of high rollers (ie millionaires) go to this gym. It's on the 50th level or so of a business tower. Annual membership is around $5000. The personal trainers there make a killing.