@ everyone: +1, thanks a lot for your input! :)
Obviously this issue requires more study from me. I've kept an eye for other human player arenas in my league and I've regularly checked out the attendance/arena expansion tool in buzzer-manager.com. The only real issue is the difficulty on making a choice, since you can't destroy arena seats and once it's finished expanding, it's really finished. I'm still striving for the total of 20550 seats since many users seem to agree on a soft cap around there.
The reason for why I started the thread is that I've read, understood and tested with my arena, that the lower tier retains more fans even after losses - and it's a rocky road to the top. Furthermore, at one's D.I league 18$ for bleachers is "normal" (from what I've seen). If you price your lower tier, say even at 21$ and succeed at filling those seats, then you're making more than what the bleacher seat could ever bring you.
@Phyr: Perhaps with correct pricing, one could influence the fans by raising the prices in the bleachers and keeping the lower tier at minimum? This of course will require some trial and error, but perhaps this way one could keep the arena at around that 12000/7500 D.I range, nevertheless of demotion/promotion issues. I assume the calculations you had on the offsite were a matter of diminishing returns, but one can only be that flexible in arena building!