I have one of the fortunate (and few) positions that pays me a salary to work in the fitness facility.
I dont get paid for training sessions, as that is included in my job description, but the salary and benefits, guaranteed paycheck are well worth it!
I train and coach on the side on my own to make some extra $$.
If you are looking to work in a gym, many gyms will pay you for "floor supervision" hours where you will walk the floor, help people out, give orientations and try to sell training. You'll then get more $$ when you are doing actual training sessions.
You'll make more money training in a studio or training on your own, but only get paid for training sessions.