I think its a case of adjusting to see what works.

I find shooting the price up after the last game of the season, then again on day 1 or 2 seems to keep the fans.

I'm at level 17 and charge $55 for LG, CL and Cup and get around 255,000 per game in my 260,000 stadium.

Friendlys I charge $15 but only get around 130,000 fans.