I hope you booked this with an agent so they can provide the exact information to you as you often can't trust the cruise line if you book directly with them.
Second, it's not unusual for a cruise line to provide a percentage off the cruise fare only. Keep in mind that the advertised rate is actually comprised of two fares; the cruise fare and the port fees, also referred to as non-commissionable fees. For example, an advertised rate may be $599, but the cruise fare may only be $439 and the port fees may be $169. So if you're offered a percentage off, it'll be a percentage off of the $439 and not the $599, and you still have to pay the $169 in full.
Hope that makes sense.