Match revenue is your main source of income. My stadium capacity is currently 35K which I can only really max out if I get a home quarter final cup tie. League matches, the attendance is effected by your current form & I average out around 18K. I've had to set the ticket prices high so I can generate the max revenue from the matches even though this annoys the fans.
It's a very tough balancing act to ensure you generate enough money each season to keep the best players, attract the sponsors & invest in the infrastructure.