It comletely depends on which day, time, whether it is a Port day (cheaper) or Sea day(more expensive). If you have already booked your cruise, go to Manage my cruise and then plan activities, Spa Treatments, select which day you think you would like it and then you'll be given a choice of items (couple massage is one of them). Only then will it give you a price. It ranges from $280-$400.
Also when you get onboard the Spa will have specials everyday that will be advertised in the FunTimes the night before.