Dude your math is way off. Like you sad not a personal attack but it just plain incorrect. The best way to raise your revenue is to promote not build the arena. Just look at how large my opponents in the finals arena is and how long he has spent building it, yet I am sure I will make more money then him from my arena next season by spending my resources on players that helped me promote instead. And my arena is literally half his!
And unfortunately you'll find out that your arena is in no way sized to help keep you in III, especially once the promotion boost wears off. The simple truth is you need to invest in the arena so you can maximize your revenues when you do promote. Players depreciate in value over time, but an arena seat, once built, never costs another dime and can bring in more money in the long run the sooner it's up.