It's difficult to know what your attendance "should" be, because there are so many variables: price of seat, type of seat, how happy your fans are, how good your PR manager is. A change in any one of these areas can have radical effects on your attendance. As you discovered; you don't know if the increased attendance is due to lower prices or the team being better. And I'm sure randomness is a factor as well.
What I did was look at other teams in my league and my level and try to get some norms. How many bleacher seats do the best teams sell? How many do the worst teams sell? What about teams who significantly raise or lower prices from the defaults? And hey, if I win promotion, what about teams at the next level? Every team's Arena page and recent game figures are available, so you can do some research. (And don't forget the "league averages" section of the Economy page.)
After you've done that, you can do some cost-benefit analysis on changes. For example, your price change raised your income by about $4 500 per game:
Game after price change: 3173 fans x $9 = $28 557
Game before price change: 2408 fans x $10 = $24 080
This may seem obvious, but I've seen teams make changes to their prices and not notice that they were making less money. If you lower ticket prices from $10 to $9, your attendance has to increase by 11 percent (10/9) to make the same money. If you're very near your capacity, lowering ticket prices is probably a bad idea, because there aren't enough unsold seats to make up the money you lost by lowering prices. But you have a lot of unsold seats in the bleacher section, so this move works for you.
Of course, that's based on only one game, and may ignore other factors. We have to work with the data we have. It's important to keep a sharp eye on these figures, to make sure the 25% increase in bleacher sales isn't just short-term fan happiness. If it is, your price decrease may not be profitable long-term.
You can do a similar analysis for the cost of arena expansion. Adding one new bleacher seat costs $200. If a bleacher ticket costs $10, it will take 20 home games to recoup the cost, assuming it is sold for every game. 20 home games is almost two full seasons. The other seat types pay for themselves at the same rate, assuming default ticket prices. So arena expansion is a long-term investment. It takes awhile to pay off, but it's worth it when it does. And if you can sell the seats. Getting promoted helps.
It's also important to consider demand. If I have 100 courtside seats and I sell 75, what is the demand for courtside seats? 75. If demand was higher, I would have sold the seats. But if I have 100 courtside seats and I sell 100 courtside seats every game, what is the demand for courtside seats? I dunno. I do not have enough supply to meet demand. The demand for courtside seats could be 101, it could be 50 000. Selling all seats tells me I failed to meet demand. Which is why I err on the side of overexpanding. I'd rather have unsold seats than turn fans away. If you sell 100 courtside seats and 150 people would have bought them, you lost $5 000 ($100 default price x 50 unsold seats). So not expanding can cost you money as well.
I like to be at 90-100% of capacity for each section, hitting 100% for the best games. That tells me I'm meeting demand. If you look at my arena page, you'll see that I'm consistently selling out bleachers, and close on the other seat types, with pretty much default prices. That's what I want.
Not that my approach is the only right approach. The moral of the story is that your arena structure is something you need to work out on your own -- there is no optimum formula that fits everybody.