It generally doesn't matter that much. If you miss the cache you are going to hit Google's network, which will serve from their own data center over their own networks. That mostly isn't paid for by the ISP, but Google's customers. The problem with wireless is that the wireless part isn't Google's network nor "fixed" infrastructure. So it becomes costly for the ISP.
Wireless towers are fixed to the ground just like the fiber they rely on for internet connectivity.
Google and Netflix peering boxes mean that ISPs with peering boxes at the same peering point can download as much as they want, within reason, for free from that peering box.
You might be confusing CDNs with peering boxes. As might I.
