[lagudragu]

You're forgetting another large cost-sink: R&D and prototyping.

Taken from their developer page (https://atap.google.com/ara/):

More Space. New Ideas. The Ara frame contains the CPU, GPU, antennas, sensors, battery and display, freeing up more room for hardware in each module. We are looking to module makers to create technology never before seen on smartphones.

With 3D printers, cheaper cheapsets, ..., tinkerers can go on and try to create their own module which they can integrate into their phone. If the module is popular amongst users, integrate it either as a standard module in the ARA store (a 3rd party could for example propose to manufacture that module, shifting those costs to them, Google would just deliver the core ARA product).