I think it's important to remember the payback period for arena expansion, too.
If you spend $200 per bleacher seat, at $10/seat it takes 20 games just to make back your money. That's about 2 seasons worth of games. And if you don't sell all the seats you build, even longer.
So sometimes it's not better to spend that money and wait 2 seasons for it to trickle back to you, especially if you can do something better with the money now.
If you are selling every seat, then it would be good to add seats/raise prices because you'll sell well after promoting--at least for a while.