I am talking about the USA and the standard commission is 17% plus other goodies that =cash. For example Royal Caribbean(RCL, Celeb, Azamara) on top of the commission gives out something called TC's that can be used by the agent for personal use for their own cruise or kick backed to the customer in the form of cabin upgrades, a bottle of bubble wine, onboard credit, etc A lot of agents may say it is 10 to 14% because a Master Travel agency may take a few percentage points from the booking.

FYI..Cruises are one of the last surviving product travel agents make money. Airlines are almost completely gone including First travel. Hotels make agents fight for their commission. Once again you can do a lot better than Amex.