If St.John's were to construct a new on campus arena it should seat 8,500. Bigger games could be played off campus at MSG or Barlays.
Monmouth's new 4,100 seat arena cost $57M. Someone may be able to provide a reasonable estimate of what an 8,000 +/- arena would cost.
Does Monmouth have all chair backs? Binghamton built a beautiful arena in 2004 that seats just over 5,000 for basketball and close to 8,000 for concerts for 33.1 million (Appx 45 million in 2015 dollars). Of course construction costs are more expensive in Queens. I would guess between 75-100 million depending on its configuration.