a personal trainer at a chain gym, where you're an employee of the gym, makes very little
an independant personal trainer can make a lot more once you build up a client base. but if you're sick you don't get paid, if you go on vacation you don't get paid, holiday no pay. if your client is sick you don't get paid, if your client goes on vacation you don't get paid. your clients often have regular jobs, so that means you're working early in the morning, then again late at night. you also have to pay for your own liability insurance and health insurance. there's no retirement plan either.
so the perfect job would be one where you sit in an office for 8 hours a day and get paid a nice salary with benefits