So is £50,000 the going rate for a day of someone's training? Seems pretty outrageous to me.
You should see what people get paid for speaking at corporate events, for a lot less time.
Remember, for events like this you're not just paying for the instructor, you have to take into account the cost of
Joe + 2 assistants
NAPP Support crew (I counted 6, possibly more)
Day rate for Models
Cost of grip hire / shipping
Travel costs for the above from the US.
Food and Accomodation
Hire of the Venue
The same will be pertinent for the Zack Arias trip, he's got to cover
Travel, Accommodation, food, all those usual expenses.
Hire of venues.
Hire of models.
Shipping or hire of large equipment.
Plus, he's also got to cover the wages bill for himself, whoever travels with him and the studio that he's not working in when he's over here.
So, pull all the expenses out of 7.5K for the day then revisit it. He's running a business, not a charity.