Quals don't determine session fees, it's location. PT at Fitness First Melbourne Central will cost more than FF Shepparton.
And half your income goes to facility rent (commercial gyms) or as a straight percentage (community gyms).
The real income limit to PT is hours. Fitness is basically a part-time job for 80% of the people in it. Because who has money to pay for PT? Mostly it's people with Mon-Fri 9-5 jobs. So 80% of PT sessions happen 6-8am and 6-8pm. Unless you spend 6am-9pm in the gym every day this limits your PT hours to about 20 a week. Some trainers get up to about 30hr but they don't last long, you just get burned out, sick of being in the gym all day, and tired with all the personality changes you need from one half-hour session to the next, like you can't talk to a 65yo retired woman the same way you talk to a 22yo male student.
You can do gym floor hours to get a bit more income, but most shifts are available at the busy times for PT - you need lots of trainers on the floor Monday nights, not many Sunday morning - so you'll only squeeze another 10hr or so out of this at most. Most shifts are done by newbie PTs with few or no clients - yep, it's inexperienced newbies introducing inexperienced newbies to the gym.
There are group fitness classes, but remember the trainer participates in those, apart from a few women with eating disorders (and you do get a lot of them being PTs) nobody's doing 4 hours of BodyPump a day, so the limit here is 5-10hr a week. And again, most of these classes are at busy PT times, so again it's either/or.
There's also front desk customer service roles, and lifeguard if the place has a pool. Again, most shifts will be at busy PT times.
Management really boosts the hours because there are always more meetings and KPI reports to do. So there you get a salary, or else you take a chunk of other trainers' session fees.
You can open your own gym, of course, and take 100% of the fees, but... then there is normal rent, utilities, etc to pay for. Barbells and that don't cost much, but if you want treadmills and shoulder press machines - easy-to-use stuff so you can collect membership fees from people without spending hours with them - those are like $5,000 each, which adds up quick. And if you have stacks of them then you have to have gym floor staff to clean and maintain them, which costs money, etc.
More trainers are moving to small group stuff. Most people don't need one-on-one past the first few weeks. Some people want it - the "personal" is more important than the "training" - but most don't need it. So okay, instead of 1 person @ $100ph let's have 4 people @ $40ph. And those 4 people will work harder and have more fun because they're in a small group. A bit more work for the trainer as you have to come up with stuff all 4 can do, but also less work because you don't have to try to get everyone into conversation, they just talk shit amongst themselves.
But the real money in the fitness industry isn't in actually training people. It's seminars and DVDs and certificates and "online training" and meal plans and so on. More people reached with fewer hours. Not many gym members will splash out $1,000 for a weekend seminar, stacks of trainers do it. Poliquin charged $7,500 for a 5-day seminar in Australia and he didn't even come, he sent a flunky. Dan John charged $2,500 for a weekend, Klokov $1,500 I think it was. Those guys all have names, but two guys from Starting Strength came to Australia without Rippetoe and charged $600 for a day workshop, there are endless kettlebell certificates from groups and people you never hard of, etc. Obviously there are flights and accommodation to consider (though they often shack up for free with the host gym owner), and lots of work to prepare for the thing outside that weekend, still it's not bad for a weekend a month.
The fitness industry is very much like the hospitality industry, I know because I've been in both. It has a low barrier to entry and is composed mostly of young single part-time casuals who will not be in it two years from now, and the real money is not made by the people actually producing the end results.
So yeah, location and hours are the real things affecting income.