Yes this will probably happen. It's just a cheapo GSM chipset. If you're buying these from Qualcomm (among others), in the millions, they are around $10 a pop (that's with LTE) The cost of a fi module will be just a few dollars more.
I'm currently using a Verizon phone, as their oligopoly results in very good coverage (It works way better than my appalling comcast connection in San Francisco). Since Verizon will likely not carry Ara, I will be looking for a new carrier as well.
If I were Google, I would give you a module for free. Subsidizing a module will cost much less a $700 smartphone (especially if you build the module yourself). This will actually increase the
ARPU, making Ara more profitable for any carrier. Fi currently gives out this swag "stuff" when you sign up:
image: zdnet looks very Shenzhen-esque
Since google will be building GSM modules at scale, we can swap out the chip compatible with Fi, then the cost per module should be much less that the items above (my guess: headphones $2, battery $8, case $1, labor/packaging $1).
To take this idea further, because of Google's business model, in a few years it may be economically feasible to give Ara devices away for free (especially as we enter the age of LTE direct chipsets). Colleges generally have a fiber optic internet connection, students should be able to access that and set up their own Wi-Fi/LTE networks. But thats a few years away...