Probably this has been mentioned before, perhaps even done to death, but there's a way to boost your finances that is far too easy. If you use your reserves and U18s to arrange friendlies against the B teams of Spanish clubs in the top division, the attendance is unrealistically high (region of 10-35% of average first team match attendance).
They always accept and even a conference south team can make an extra 30-40k/month.
Also seems to be a bug that prevents you arranging frienlies with Sevilla B when you have the spanish primera liga running, not sure why that could be.