Probably same as real estate... what ever the service is worth.
Many people "think" real estate agents charge a fixed fee but that would be illegal in the US.
Boat/ Yacht Brokers charge depending on what services they are providing and what expenses they have in representing the yacht.
I personally know of significant variations in several locations in the US and outside the US but they all basically charge for amount of service provided which will be different for different types of clients and boats.
10% does seem to be the going rate. When we bought our 8 year old Dehler last January, the broker did drop his fee a bit to make the deal happen when we let everyone know this was as high as we were going. I have been told as the cost of the boat increases the more willing a broker is to take a smaller commission.
10% has been standard 'forever' but today everything is negotiable.
The less work the broker has to do, the more likely he is to be willing to bargain on that. And if two brokers get involved...just like real estate, that may become a 5+5 split, with neither looking to give up anything more.