Here are the basketball attendance figures:
http://fs.ncaa.org/Docs/stats/m_basketball_RB/Reports/attend/2016.pdf
Top 10 teams in average home attendance last year:
1. Kentucky 23,361
2. Syracuse 21,592
3. Louisville 20,859
4. North Carolina 18,326
5. Maryland 17,863
6. Wisconsin 17,287
7. Indiana 17,106
8. North Carolina St. 16,758
9. Kansas 16,436
10. Creighton 15,941
I think there's a significant revenue boost to Duke. Home teams get to keep all of their ticket revenue, but for neutral site games the teams usually split the revenue. While MSG is technically St. John's home arena, I bet Duke has enough leverage to get them to split the gate. After all, Duke would be the big draw in that game.
As for capacity, MSG holds 19,812, Carnesecca holds 5,602, and Cameron holds 9,314. The average price at Cameron is about $200 (source:
http://247wallst.com/special-report/2016/03/10/the-25-most-expensive-college-basketball-tickets/), so a home game at Cameron with 9,000 paid tickets is worth $1.86 million in ticket revenue
Let's say you are going to schedule a home and home with St. Johns. You can give do one game at Carnesecca and one at Cameron. That gives you $1.86 million in revenue at home and nothing for the Carnesecca game. Even if you could get a 50% split of the Carnesecca revenue, selling 5,500 tickets at $60 per gives Duke $165,000.
However, if you can get it moved to MSG and sell say 15,000 tickets at a higher ticket price (say $100 because of the location), Duke's split of the revenue is $750,000. So Duke gets $585,000 more than they would at Carnesecca, and St. John's still gets $420,000 more than they would for a $60 per ticket sell-out at Carnesecca where they keep 100% of the gate. It's a win-win for both parties.
This is why you're seeing a move in college football and basketball towards neutral site venues. It's tough to make the economics work to play road games for the top teams. That's why I really liked our move with Davidson to play a home game at the Dean Dome in 2016 and a neutral site game in Charlotte in 2017, rather than a return game at Davidson. It results in more revenue for both teams, and one could argue it's a nice opportunity for UNC fans in Charlotte to see the Heels play as well.