Grab small servers with one or two big disk (Hetzner will give you two 4TB disks for 39.00 EUR, OVH one 6TB for about 30 USD w/ an ARM CPU, less than 50 USD for two 4TB under the soyoustart brand), serve them over iSCSI, use RAID-Z3 to create a nice huge ZFS filesystem. Do not forget to have hot spares. Done. Costs about half or even third of what S3 costs and it's a filesystem instead of an object storage. Also has transparent compression. ZFS grows online. Both OVH and Hetzner and I bet other such providers have API so you can just script the whole thing to grow as soon as you cross a threshold.