I think it depends on the gym. Some gyms pay on a scale depending on how many clients you are servicing and packages of training you are selling. So at those gyms your pay is not tied to education. Other gyms require that you become a "master trainer" to get the highest pay, also not tied to academic education (rather it's attending those specific master trainer courses). I know of 2 gym chains who start your base pay based on academics and/or years of experience but honestly, it's usually not that much more per hour.
Trainer pay in general depends on the clients you are serving and your success as a trainer (not tied to education necessarily. I've worked with some academically smart trainers that just weren't successful at it).
The range of pay is anywhere from $25-$50/hr (my experiences in the Midwest). With a bachelors degree only and training experience, it is possible to get $50/hr. The DPT won't change that in my opinion.