That add is from Nowra Power Boats, now think of it this way, there are dealer licensing fees, overheads (rent, staff, labour etc) if they are from China there will be a fair amount of money in freight, container fees etc. but it appears these are from another manufacturer being Seatrail trailers, which will have their own markup on the manufacturing costs to the dealer, and of course the dealer needs to make money somewhere. Also you will find a lot of their sales will come from boat and fishing shows in which the organisers charge a premium for site fees.
When you break it down like that I think you’ll find that they’re not making near as much as you think they are.
Think of it the same as a taxi ride, you sit in the taxi for 15 minutes, they charge you $50, you know full well they didn’t use $50 of fuel, but then there’s servicing costs, registration, insurance and so on. If you don’t want to pay what they want that’s fine you can build your own but dont knock them for trying to make a dollar.