I know how TA's get paid and describing it as healthy commission is being generous. There are are also may factors that come into to play with that commission. It could be split between with the host agency they work for, they need to pay their expenses and it is likely their only source of income. The commission rate will also be different by cruise line and also dependent on the agreement they or their host agency has with the line
Would you ask for your lawyer, your plumber, your real estate agent, your hair stylist to give you back part of their pay/