The space behind sections 104-106 would be tighter than the space behind sections 101-103.
A row of seats across sections 101-103 would be approximately 50 seats. Conservatively, you could install 6 rows of temporary bleachers, you would have around 300 seats.
A row of seats across sections 104-106 would be another 50 seats, you could install 3 rows of temporary bleachers instead of 6, you would have around 150 seats.
So, 450 seats, let's say the 3-day strip would be somewhere around $600-700, that's about $300,000.
If you want to get more aggressive and put in 10 rows behind sections 101-103 you are getting closer to $500,000.
I think the trees you would need to move are only behind sections 105-106, so it may be more cost effective to only install the bleachers behind sections 101-104, about 70 seats across.