I'm at season 6 i took one season of at season 4 to tank, build my team and build my arena and now at lvl 6 my arena is still behind I can seat 26k and max i think is 29k for my level but now i really start notice the income from it, season 5 i made 100k per home game and now in season 6 i make 250k per game, a huge different.

Also the sooner you upgrade your arena the sooner you can upgrade your medical and training facility's, and who doesn't need more red and green boosters?