Any ideas on what it would cost to store about 6TB of data on S3 or whatever else? I don't know what tiers there are, but it's just for backups, so it can be on whatever cheap ones they've got.

It's time to follow my own 3-2-1 advice.

@north Good question I hadn’t thought of. On a lark I asked ChatGpt (grain of salt): “For ~6 TB of personal backups, Amazon S3 ranges from ~$140/mo (Standard, instant access) down to $6/mo (Glacier Deep Archive, hours to restore), with Glacier Instant ($24/mo) as a middle ground.
@north if you’re truly just going for backups, and would only ever need the data during a restore, deep archive those suckers. It’s as cheap as object storage gets. Just tar things up first to avoid the per-object fees on put / restore.
@north
Also check restore times and egress costs.
Random search for S3 Glacier costs found https://www.arqbackup.com/aws-glacier-pricing.html which describes the whole lifecycle. They also mention backblaze and wasabi.
AWS Glacier Pricing - How to Calculate the Real Cost | Arq Backup

Understand all the S3 Glacier pricing to avoid substantial unexpected charges