What's the problem exactly? Go back to your previous price, and slowly raise the price by $1 per home game until you don't get a full house anymore, and leave it there. You'll still get a full house sometimes, though. Prices can be slightly higher for CL and even more for the Cup.

It's also possible that your stadium is already too big and you're simply not allowed to have more attendance. How much bonus possession are you getting? If it is 5% but your stadium is not full, then you've already hit the attendance limit for your level.