Egress
The fee cloud providers charge to move data out of their network to the internet. Storage is cheap; getting your data back out is where object-storage bills add up.
What it is
Uploading data (ingress) is usually free. Downloading it, or serving it to users, costs per GB - roughly $0.08-0.12/GB on the hyperscalers (AWS, GCP, Azure), $0.01/GB on Backblaze B2, and $0 on Cloudflare R2 and Wasabi.
Why it matters
For a read-heavy workload - serving model weights, datasets, media, backups pulled cross-region - egress can dwarf the storage line. It is also the main lever cloud providers use to discourage you from leaving.
Example
Serving 5 TB/month of model artifacts: ~$0 on R2, ~$50 on B2, ~$450 on S3. Over a year that is a $5,400 difference on transfer alone.