I went back to look at some earlier articles and they claim that it would be in the black with 100 events a year (
http://sports.nationalpost.com/2011/11/23/what-the-is-markham-trying-to-get-an-nhl-team/), so if Live Nation can really bring 130ish concerts a year, it should at least be breaking even, even if it wouldn't be highly profitable. The ACC is the 11th busiest arena in the world (according to wikipedia), so I'm not sure it's a fair benchmark.
The town is assuming 130 events and 780,000 attendees based on concerts alone. By comparison, the MTS Centre sold 385,427 tickets in 2008, making it the third-busiest venue in Canada. If anyone has more numbers for Canadian arenas, I'd love to see them.
My point is that as a taxpayer I don't really care if it's highly profitable; I'd be fine with it turning a small profit each year.