I tend to think of expansions differently. At first I thought as you did that the higher the price the more you want of the seats. But then it dawned on me. If you sell 100 seats for $200 per that's 20,000. If you sell 200 seats at $120 per that's 24,000. So 4,000/100 =$40. So each extra seat from 100-200 = a profit of $40 per. It will take roughly 50 weeks to get back the initial investment if the seats cost $2000 each.
I don't really get your calculation, does it include the assumption that if you build 200 seats instead of 100, you need to lower your ticket price from $200 down to $120? In that case, yes the break-even point is far ahead yes, but it's also a pretty extreme situation. You don't regularly double the amounts of seat you have at once.
I have to say my theory had some assumptions too, but not as extreme as yours if I understand you correctly. My assumptions were that you don't invest huge amounts of money at once in your arena, more like max $200k each time. This way, the increase in seats is only a small percentage of the seats you have already, making it less likely you'll have to decrease prices right away for the seats to sell out. I already said that the return on investment only goes for seats that are selling out.
The point of the predictability of the more expensive seats is a valid one, though in my experience in the lowest divisions you'll quickly reach the point where you beat the majority of the league easily, making the bleachers predictable too, since 90% of the team you'll come of a win before a home game. But, if the Return on investment is roughly the same, you could use this point to decide to build the expensive seats first.
And if you reach the point where you can't sell many more seats because your arena is already much bigger than the opposition's, then you might want to prioritze strengthening your roster over your arena, but again that's an exception situation. I appreciate you adding these points to keep in mind, but I was just giving a simplified version of how I think about expanding your arena to help this guy out.