seats always cost the same, regardless of league.. but remember, they take time to build, so if you do it now they are ready for the start of next season...
you have the cash to do it, so why not... the quicker you do it, the quicker you can start getting a return on your investment.. The money in your account doesnt accrue interest, so i would say it is costing you money by not investing now.
Again, i suggest, do 500 bleachers at a time... by the time the new season rolls around, you will have 1000-1500 new seats which will be worth an extra $9000-$13,500 per home game at your current prices, then focus on some lower tier. take a look around some US div 4 leagues and check out there arenas, and try to model on that.. also check their prices and copy those.. i think ur prices are to low.