Hmm, interesting. I have never had money struggles to be honest after the first five levels. But I win at least 1-2 titles every season so that keeps me in the game. Fast ways to earn some money:

1) Watch adds for money. I prefer green packs, but if you are struggling then you can easily earn 100 mln or so per season
2) Test out how high your ticket price can go. Lots of times you can still increase it a little. Make sure your stadium is at max as well as your other facilities but at lvl 26 that has to be the case.
3) Play friendlies if you are out of the CL or Cup to still get some income (and some extra packs to)
4) Join an association for extra games and some extra play money.
5) Train a player to 79/99/119% and sell him only then and hope for a good fight for him or get a high bid through negotiations
6) Use tokens to get money. Yes, it is a waste but it can be a last resort.
7) Use the daily contract for sponsoring as it brings in the most money.

That is all I can think off right now. Cheers.