It was $345 per person to add airfare and transportation from airport to pier to our cruise package. I found airfare for $179pp. $20 taxi ride to the pier each way. But.. you are on your own if there is a delay. It's a gamble.
The advantage is that if your flight is delayed or cancelled it becomes NCL's responsibility to get you to the ship, if you have booked through them. If you book on your own you have to scramble to make other arrangements and pay any extra yourself. If you miss the ship due to airline delays NCL has no responsibility and you are just SOL.
In the past we have addressed this problem by flying in the day before and staying somewhere inexpensive over night. That gives us enough time to deal with delays, if they occur, and still make the ship in time. It does add extra time and cost to the vacation. In my case I usually book airfare with airmiles so it is still cheaper for me to do it that way. If you book on your own and for some reason NCL cancels your cruise, NCL should refund you what you paid for the cruise but will be left with your air ticket to deal with yourself. If you book through NCL they should refund the air portion also.
On the other hand I have read several times that NCL air arrangements are often strange involving really weird routing and long layovers etc. I haven't experienced this myself but have read several complaints on various boards.
I ended up booking the air myself. Saved $640!! We will be flying in the day before. NCL said the only other thing I will miss is the transfer between the airport and the pier, which I don't need anyway.
I'm not positive on this, but I think I read that sometimes you can get a better deal on an "open jawed" flight from the cruise lines. This is one where you fly into one city and then out of another, for example Miami-Los Angeles via the Canal.
In my case, the flight cost was actually a good deal. I wasn't planning to book flights with NCL, but it was a good price, and now I have the ease of knowing if we are delayed or have problems, NCL has to manage it, not me...