Why would it necessarily be cheaper? After all Apple has both economy of scale and the capacity to buy up the entire manufacturing run of many of the components

Why would it necessarily be cheaper? After all Apple has both economy of scale and the capacity to buy up the entire manufacturing run of many of the components. They should be able to assemble a system as tightly integrated as a Mac much cheaper than somebody could go out and buy all the parts for. A better example might be the cost delta between buying a car from the manufacturer and buying all the parts to make the same car separately and putting it together. The direct from manufacturer car will be far cheaper.