Every cruise line will get the highest cost per cabin possible. Celebrity's brochure rates are generally higher than other cruise lines, but their service and food is much better as well. Occupancy is highest during the holidays, so they can get higher rates. It's very similar to airline pricing today.
Otherwise, everything seems to be selling for $599 these days
That's because of the enormous number of new ships entering service each year. Supply exceeds demand.
If you're set on Celebrity, bypass their newest vessels, which always sell for the highest prices because they get the highest demand. Zenith, Horizon and the Century-class vessels are just as wonderful.