What is the max arena size? Is it 28,000?
Also is there a max on lower tier seating?
You can build a huge amount of bleachers but there is a soft cap of 20k on attendance (really hard to get much more than that).
Also is there a max on lower tier seating?
I'm not sure but if there is it is high enough that you don't need to worry about it.
I would like to have 60% ticket prices.
I dont know the arena size that would make these prices worth it, your numbers seem at least close for a D4 promoting team.
But I don't know why you would want these ticket prices. If you have to set your prices at 60% of default that means your arena is "too big" for your division. You can't really have a too big arena but what it means is you are getting almost no return on investment for much of your arena expansion; you could raise prices, sell less tickets, and lose hardly any cash.
You are still a ways away form the arena size you would need for 60% prices to make sense next season. You already have a pretty good arena. Instead of spending all your cash upgrading your arena, I recommend looking for some truly salary efficient players in the 10-25k salary range so that you can make more money and do well in D4.
This advice is coming from someone who built an above average D2 arena from nothing in under 4 seasons, so I know the value of arena expansion. But there is such a thing as too much.