The $50 deals change often, I assume they pop up as others are cancelling at the last minute. I research the cruises I want in advance so that I'm ready when it appears. I refresh the deal page often throughout the day, lol. Also, I've noticed the cabins are usually for two people, haven't had a chance to add a third to any of the cruises I've seen. Because of this once I see the one I want I click on the price listed under cabin type which takes me straight to the booking page vs clicking on "start booking" because the cabin will be gone by the time you go through the additional steps.
I was able to grab a $50 8 night Panama Canal cruise in March. I currently have the same offer, almost booked the Celebration for next week but I want to cruise later this year.
Good luck!