The commission is paid by out of the yacht charter fee to the yacht charter broker. Sometimes the yacht will give a discount on a week they want to sell or for repeat guests. Same with a broker.
They may offer a bit off their commission back to repeat clients or add in extras that the yacht does not cover. The commission rate is fairly standard throughout the industry. The yacht pays 15% to the broker on average, sometimes a bit more or less, depending on the week. It could be a bit more if you have a Travel Agent you work with as well as the Yacht Charter Broker.