You can make good money (I do) but generally not at the beginning and not working for a chain gym. Right now I'm stuck working for one because I just moved to this city and they provide lots of access to people. More people = more clients faster.
I started at my gym for two months and had a full roster - meaning I'm training 30-35 paying sessions per week (including classes and my running group). That is where I cut myself off because working more than that is stupid in this business. I have a small waiting list starting as well and I'm starting to build a good reputation, but it takes 6-12 months and then I'll go out on my own like I was before, which is where you make much better money. Right now I'm making chump change compared to what I was, but it's necessary due to being in a new city. I know that will change. I was formerly making about $1000 a week working 20-25 hours actually training. Not bad for that amount of time invested - sometimes I worked 2 hours a day, sometimes 10. Right now a typical day is 5am-11am and then 4pm-7pm but I'm working more because I'm making far less/hour.
The difference however between me and the typical gym trainer is that I have several high level certs, ten years of experience and I'm very specialized in injury management and endurance sports - something noone else in most gyms have because all they care about is bodybuilding. Meaning, when people walk in who need these things - I'm it. Even the other trainers refer people to me because they have no idea what they are doing. I also give excellent service and I'm 100% focused on my clients when I'm with them - other people at the gym really notice that.
So can you make good money? Yes. Does it take hard work and a lot of commitment to your clients? Yes. Long crazy hours? Yes. Stressful cycles of income? Yes. No paid vacation? Yes.
Oh, and you make no money for the first 3-6 months while you are building your client base. That sucks.