I beg to differ.
I have kept very close tabs on the cost, hoping for a price drop. I used CruisePlum as well.
Since last June, there has been no price drop on our Jan Panama cruise in a Family Balcony cabin, regardless of the numerous 1/2 off, 2nd passenger free or whatever else they have had on sale. When I do the trial booking, the price has been identical.
So, I am quite sure the price does not go down