That's the $64,000 question. I'm sure the cruise line exeutives know to the penny, but they'll never say.
Cruise prices fluctuate just like airline seats. It's based on supply and demand and cruise lines, airlines use a program called "Yield Management" to set prices, which may vary by the minute. Carnival has 43 people in their yield management division.
A friend called, thrilled she found a fantastic price for an Alaska cruise. I said "run, book it now"!! Three minutes later it was $200 higher at the same web site. She knew enough to grab that!
But in the end, cruising is still the best vacation value you can find. You pay for 80% of your costs upfront. Who wants to worry how much it costs to dine out each evening?