What costs is having a CDN, a bunch of very fast servers that exist in every point-of-presence, given the load they endure CDN cache servers fail quickly when compared to others. -- along with the upkeep of their networking equipment, which is cheap but not free.

But Google itself has invested wisely in how it connects to the internet, they are dark fibre all the way with many hundreds of gigabits between sites and pops. It's a huge upfront investment (the kind SV startups seem to hate) but the long tail makes bandwidth essentially free.

The only cost they have is hardware and peering, and given their size I can't convince myself if they are or are not being shafted financially by big ISPs for peering - even if they are though, it's marginal compared to what GCP/AWS/etc; charge us, even Colo datacenters will charge significantly more than what it costs Google.