The way I see it, you paid what you considered a fair price for your cruise. If you didn't think it was a fair/good price you would not have paid it. Would it be nice for NCL to give you a discount? Sure, but they are not obligated to do so. They have lowered the price now in order to fill the ship, if everyone already booked got the bargain basement new rate then the cruise would probably be a loss for NCL.
IF you were before the penalty phase of your booking (90 days) you could simply cancel the booking and rebook (at the risk of not getting the cabin type you want) at the lower rate. But since you are within the penalty phase, you should look at it in the light of having gotten a fair price. Next time, you can take the risk of waiting until the "last minute" hoping for a cheaper rate, but be warned that your desired cruise date may be full by then too.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013