Welcome to cruise-chat. Many of your questions are answered on NCL's Web site, located here
. I'm not trying to pass the buck -- but there's a lot of information there that we don't have the space to summarize here.
As the others have mentioned, I highly suggest that you use a cruise-certified travel agent for your booking and to make other arrangements. I doubt that getting off the ship permanently along the way is possible. Depending upon the itinerary, the Passenger Services Act may prohibit it. (This would apply to any cruise line you might be considering, not just NCL.) Again, this is something that a competent travel agent can discuss with you.
As for your initial question about prices -- a lot depends upon the size of the stateroom you select, as well as other factors. Either way, it's a mighty expensive way to get to Jamaica, if Sandals is your primary reason for the trip.
Good luck with your planning, and congratulations on your wedding.