Do the prices go down a bit the closer you get to departure, or does it sell out pretty far in advance??
The cruise is usually fully booked a year in advance...I doubt you can find a cabin for 2018. I tried a couple of years ago and was looking for a spot about 12-months in advance and was not successful.
I have not checked on availability this year, but my guess is that you would have to look for something in 2019.
The prices are based on what cabin you choose and I do not believe they offer any discounts. They seem to have a pretty full standby list for any cancellations.
Good luck!