Using Pollstar's 2014 NA report -
Britney's avg per show - 3,995 attendants w/ an avg price of $150.
3,995 x 70 dates in 2015 = 279,650 tickets x $150 avg price = $41,872,500 for the year.
279,650 tickets out of 322,000 tickets (4,600 theater capacity x 70 dates = 322K) = 86% sell out rate for the whole year, which I think is reasonable. This should get the show to nearly $80m gross. She's sure to pass the $100m mark with the 2016 shows.
