how much is a good amount to spend on your arena at a time?
Here's how I decide if I need to build up my arena and by how much:
1) I look at my last game. Has any of the seats been filled up?
2) If yes, build 10% more of it or however much your money allows.
3) If no, do nothing.
4) Next game, repeat.
I'd like to hear a better way or how other people do it.