a bitcoin miner isn't a isn't a general processing unit.It does something specific that's very very easy to optimize and implement, and you don't use latest manufacturing process but tools like easic , hence the small investment.
The money is a huge barrier to entry for hobbyist types, but if we're talking about commercial stuff, it'll probably cost you a lot more to pay for the engineers who are competent enough to implement something that will actually work than it will to do the fabrication.