You have similar number of fans as I do.

I'm level 24 and all max level facilities, with 6.03M fans.
I charge $50 league, $54 cup or CL and $20 friendly against mourinho.

As your stadium isn't max I believe you can charge something around $52~55 or even more and still get a full stadium.
Try adjust ticket price with increment of $1, starting from $51 for league and $55 for cup/CL.