What does "free" look like?
e.g. the least possible definition I could imagine is, base cruise fare for a 3 nights on their oldest ship during the low of low season, inside cabin, one traveler. Perhaps sort of premium if you want to bring a guest and of course taxes, fees, tips not included.
Of course it could only get better from there.... if you can bring a second person to occupy the stateroom and with fees and taxes and whatever else they want to add, it is still substantially less that what you'd pay with a normal booking, and it isn't necessarily on their oldest and worst, then it starts to become attractive pretty quickly.