The cost is mostly in the tooling. So, if you are doing 1 million units and running your tooling 6000 hours per year, then if the cycle time is less than 20 seconds for any operation, then you are not fully using that piece of equipment. If you are doing 15M units a year, you will need to design/build/buy 10-100 units for each manufacturing step. That cuts your cost tremendously, because the custom design and engineering only needs to be done once.

Apple has been known to build the factories for the suppliers or at least contributing to the cost to secure the products coming out of said factory. I remember them building a factory with Samsung (ironic) and Foxxconn.