If you scroll a bit on the Backblaze B2 pricing page, they have an easy calculator that will tell you exactly what the costs will be.

$0.005/GB/Month for storage

$0.01/GB for egress

The only "hidden" cost I'd say they have is they have a limit on some API calls. For example their b2_download_file_by_name is limited to 2500 calls per day and then $0.004 per 10k calls after that.

https://www.backblaze.com/b2/cloud-storage-pricing.html

https://www.backblaze.com/b2/b2-transactions-price.html

https://tebi.io/ charge for the storage, you can define how many copies and where you want to store your data and a flat egress fee

Filebase is S3 compatible and has pretty straight forward pricing [1]

[1] https://filebase.com

Digital Ocean spaces has pretty straightforward pricing?

Hetzner storagebboxes/shares are great value - but no s3 api.