I have a full capacity stadium 260k i cant remember how much it costs from 0 to 260k, but i remember i payed around 400M for the last upgrade .
Totally worth it, i get 260k full audience every game.
With 65 dollars price per ticket i get 16M income per match + 5M other goods, and the competition prize either is cup, CL or league.
With two matches everyday i get a 50M income daily.
I don't even bother selling old players or duds past 26 years, i just sack them.
My advice is to quickly upgrade stadium.
I am currently lv23 and i already reached the 2.15B money cap.