AWS just yesterday launched flat rate pricing for their CDN (including a flat rate allowance for bandwidth and S3 storage), including a guaranteed $0 tier.

https://news.ycombinator.com/item?id=45975411

I agree that it’s likely very technically difficult to find the right balance between capping costs and not breaking things, but this shows that it’s definitely possible, and hopefully this signals that AWS is interested in doing this in other services too.