In my experience with the Chinese (of which I've had a fair bit), they are very, very good at reverse engineering hardware. So if you give them a box of bits, they will make the same box of bits very cheaply.
What the Chinese are not good at is designing stuff from scratch - why design it when you can copy it? They're also not very good at reverse engineering the software that runs the hardware. First of all they have to identify the microprocessor that's used, which is most likely manufactured exclusively for ML with its own part number that is useless to anybody else.
I would also expect that ML have taken precautions that will "brick" the microprocessor if someone tries to hack into it to retrieve the software. I think at best, the Chinese could make a chip that would emulate the controls and LCD display on a 5000, but I doubt they actually do much.
The Chinese philosophy to making money is "If I only make $1 from every sale, I only have to sell 1 million to be a millionaire". To put this into perspective, when you have a population of 1.5 billion people, selling 1 million of something is not much of a stretch. So if it costs them $50 to make something, they are quite happy to sell it for $51 as long as they can sell a lot of them. But guess where most of the stuff from China goes to - yep, the USA. Trillions of dollars of crap gets unloaded on the Americans who are brainwashed to "just keep buying stuff". Australia is a mere pimple on the butt of the world economy.
China is now very much a "capitalist economy". There are more millionaires in China than the rest of the world put together - they simply love money and to a China man it is very important to be seen to be successful. Losing face in China is not an option.
As for joe's question - a Chinese copy plant probably can't understand the all of the complex design drawings, even if they did have them. More than likely they would've just used them to find out what types of material they need to use to copy the parts (e.g. coil materials, control box materials etc.)
Toysandthings - I agree that the parts to make up a GPX 5000 would be way under $1000. But what you paying $6,500 for is all the R&D, infrastructure and overheads that all add up before you even buy the first screw to put it together. And of course, there is some margin in there for dealers and taxes on top of that. The other thing is that as long as people are buying them, the price won't drop anytime soon.
I would not at all be surprised if the new Garret detector (that will rival the GPX 5000) retails for just under the price of a GPX 5000. Neither company wants to start a pricing war that will endanger their profits. It's not good for (their) business.
AU