I've been building my lower tier for about 3 seasons now and completely left my bleachers around 11800. But when they're priced at $20 and they're full every, single game I finally decided that I needed to build more this week.
I definitely don't think that 10000/10000/500/50 if the optimum number as the guys that have that many lower tier built can't fill more than 7000-8000 of them at the lowest price point. At the same time, I don't think that 15000/5000/500/50 is the best number either. I think the perfect number falls somewhere in between those 2 sizes. Right now I'm leaning towards something in the 13000/7000 range although 12500/7500 might be ideal too.