Blindly? I would guess somewhere between 3 and 4 mill fully staffed. But i have nothing to base that on other than the fact it cost a billion to build. In terms of revenue we are talking about a number many times that.
Logically, and based on some quick research I'd guess cost for Metlife is $5-$10/seat * 80,000 seats = i.e. between $400,000-$800,000. There is probably not a lot of competition for an open stadium on a winter day.
In terms of revenue, If you assume average seat price at the siyum is $75 * 80,000 = $6 million in revenue, so they should walk away with a nice profit, even after heavy marketing.
It's hard to find a better cause. They do what no one can do in Washington DC, Albany etc, and if you wanted to try to evaluate that in dollar terms, I'd guess they save the broader community and especially schools at least many tens of millions of dollars a year.