Since Great Stirrup Cay is NCL's private island the zipline will probably be listed as a ship tour and it would probably booked as a shore excursion so price is likely going to be what NCL decides and price should be on your list of available excursions.
Sorry couldn't help more, 30 years of cruising and over 40 cruises, have never been on NCL. But since it is on their private island maybe it is a free attraction (hold that thought but don't hold your breath at the same time) ..............................
Will ask around so if you don't find an answer check back if I find out anything will post it here or private message you on our forum PM
But did find this, so for what it's worth this may be the secret excursion, and is not listed as far as I could find so YUP the answer of the day will likely come from someone that has experienced it "Great Stirrup Cay's most distinctive landmark is the white lighthouse rising from the center of the island. It has been repurposed as the starting point for a massive zipline course that combines six ziplines with V net bridges, chimney climbs and cargo nets for climbing fun. Sign up at the excursion desk. "