On Princess, if you get a deal or "book" a future cruise while onboard, it is $100/pp. Also, with a future cruise, you get $100/pp cabin credit. That is all I have ever paid. Princess has specials all the time.
On RCI, it was $250/pp.
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08