My trainer is level 6 (so is my doctor btw), and he costs me around 75 k$ weekly (the doctor takes 25k and the PR representative 20k).
In my economy page I saw that firing the minimal staff cost me the equivalent of their weekly salary, so if I fire my trainer, it will cost me 75k$.
Anyway I'm happy with my economy now, it's almost at equilibrium (actually I'm making 15k$ a week with the 50k bonus), and that is with the basic arena, so it should be fine.
I'll see what my team is capable in my first real game against a full team, I'm just scared when I compare the league's average payrolls to mine, but who knows!