Current true bandwidth costs about $0.50 (50 USD cents) per Mbps, per month, sometimes even less.
Average 1 month usage of a 1Mbps link that has Internet traffic on it is about 175-210GBytes.
Literally less than 1/3rd of 1 penny, per GB of data.
From eg Hetzner and other less affordable hosts (and from second hand comments here on hn about bandwidth is where amazon has ridiculously high margins) - I'll venture that the "cloud" providers get away with it because on average "actual" bandwidth correlate with "actual" usage: and hence correlate with actual users that pay or are worth paying "for".
I get 30tb/month bw with my 50/month hetzner dedicated server (along with 1gps uplink). And I can actually use that.
I can't even see backing stuff up to a typical "cloud" provider, because testing restore of a single tb of data/month would be too expensive.
People say this about AWS/GCP/Azure. They are so willing to compete on cost on everything else that it seems odd like this would be true, so I have a different theory:
OVH/etc are all able to massively oversell bandwidth. Most customers do not use it. This is fine, and not anything I think they're doing wrong!
But the "Big 3" are all about automatically scaling resources. With my own applications, I find that if I am scaling up/out, my network traffic is growing at a commensurate rate. Whereas back in the day when I was using SoftLayer or other places where I leased dedicated servers, I was not using anywhere near the included bandwidth.
It's easy to sell 1TB at the price of 100GB if people, on average, are only using 100GB. But then you have to sell 1TB at the price of 1TB if people, on average, are using that 1TB.