If you're seeing double the price you're looking at the NY ports including Bayonne, NJ. Probably comparing NCL Dawn to RCI Explorer of the Seas to Bermuda. NCL has run Bermuda routes for a long time and RCI is newer to that market. One reason is that people want to try and new thing when it comes to town. Another is, there is more to do onboard EOS, like the ice rink, rock wall and Promenade, a mall through the center of the ship. Another reason is RCI is running 9 day itins and Dawn is only 7.
Welcome to cruise-chat, hope this helps.