I'm very much not in the know (my personal experience tops out at renting gigabit lines at 95th percentile), but it's my understanding that at extreme scale, you're doing peering arrangements directly with ISPs, and provisioning a fixed amount of bandwidth with them.

It wouldn't surprise me if Google was scheduling the majority of updates to take place in off peak times, when the bandwidth would simply be otherwise unused. If so, the cost to Google would be a rounding error of your estimate based on GCE traffic prices.