Since I'm an obsessive geek, I've scouted many teams in DIV and some III, looking both at their arena sizes and their rosters. From this, the teams earning the most money from attendance have expanded their arenas substantially. They are making more money than those teams who are relying on higher ticket prices. Since there's no maintenance payment for the arena, the only cost to upgrades is the opportunity cost of spending that money on players instead.
For me, I'm shooting for about 7K bleachers, 750-1000 lower tier, 200 courtside. I already built 10 lux. boxes, but that is too much for DV, and for DIV you probably don't need many more.
@WFUnDina - That idea about home court advantage is a good one, but I hadn't heard that.
As for why not to expand the bleachers and lower tier, they are pretty sensitive to your team's performance. So the money you earn from them can fluctuate wildy. Courtside and lux box should be less sensitive to performance (but are still price sensitive).