"Real" bandwidth that you're allowed to saturate costs an ISP about $1/Mbps/month, and that's not counting the cost of building the ISP's network itself which is probably much higher. Today's $50 broadband service is maybe backed by 1 Mbps of real bandwidth if you're lucky, but changing the advertising from "up to 15 Mbps" to "1 Mbps guaranteed" is commercial suicide.
I would bet that almost all of the cost of Google Fiber is in the last mile; OTOH Google can probably buy bandwidth very cheap. It's probably still oversubscribed 100:1 or more.
