Ask yourself how much would it cost you to design, build, and pay for all the electronic, plastic, aluminum or carbon fiber pieces, while not infringing on patents from Fisher or any other company's products, a comparable detector from scratch. Does anyone think they can do it for less than a hundred bucks in their own house? If I had to solder all the parts in the main board, I doubt I would make a dollar an hour to do it for $100 and I wouldn't have even started on winding a coil or developing the software yet. Maybe I could get Mark Lawrie, Carl Moreland, or Dave Johnson to work for free so I could at least have a clue how to do it.
It reminds me of people who think since a barrel of crude only costs $80 a barrel that gas companies are gouging consumers with their prices. I say, "Make your own". First pay for equipment to figure out where it is buried, then pay for equipment to recover it from the earth, pay to transport it to where you paid to build a refinery, make sure the government regulations are met and government taxes are paid, and then, pay to transport it to your vehicle and then fill it up yourself, saving money from having Pilot for 7-11 do it for you. Do you think you can do it for $2 or $3 dollars a gallon? Wheat costs less than $6 a bushel. Why does Wheaties and a loaf of bread cost so much?