My suggestion is to show when the arena completes expansion on the arena page under attendance history.
This way we can see how the number of tickets sold changed after completion of the arena, and we get to see how many seats were available at the time.
An example is shown below:
Attendance History
Date Opponent Bleachers Lower Tier Courtside Seats Luxury Boxes Total Attendance Game Type
2/13/2010 Tommys Guns 4500 575 100 6 5181
2/12/2010 Screaming Eagles 4500 575 100 7 5182
2/10/2010 Expansion 300 15 15 1 331
2/9/2010 Australian Flames 4500 552 100 6 5158
1/30/2010 Asian Delux 3872 575 90 5 4542