NYC is way cheaper, I fly out of Newark, but only because it is easier to get to for my parents and if I didn't they would totally make me take the bus.
As far as I know, they will not give you an extended stay past your BUNAC date on your entry, but I literally hopped the ferry to France, didn't even get off the boat, and got another stamp for six months. But I am not sure about the flight, I would think as long as you have a return and explain you should be ok.
The flexiable date for Virgin was only on the studentuniverse website, I am not sure if it is available elsewhere.