Your level is still low and it will be that way for a few seasons.

At level 21 my full season sponsor is 25.4m, 7.62m weekly or 1.72m per day.
I think the sponsorship amount is linked to how many fans you have, not your level.

These are of little use eventually as a match at home game I could sell tickets and goods for around 21m profit for a normal league game which is many times more than the prize for each match even for the cup. Without that my team couldn't be sustained... My stadium is fully upgraded.

While in doubt, upgrade your stadium.