In addition to what other people mentioned, there is also a huge cost in managing all the metadata that you'd get from billions of files. This really even worse if you're using a crazy 64-32 encoding.
We build clusters of low PB scale that have a TCO with everything from labor to hardware, from financing to electric, from routers to cables and can be run below 3€/TB. For that you can store data in Block(rbd, iscsi), Objekt(S3, swift) or Filestorage(CephFS, NFS, SMB), high available on Supermicro hardware in any datacenter worldwide.
Feel free to contact us or use our free community edition to start your own cluster.