Then how are Vimeo doing it? They're fixed price.
They limit you to 2TB per month of bandwidth:
* https://vimeo.com/upgrade-plan
If a video goes viral and you shoot past that you'll probably get a call from them.
You also have a finite amount of storage space available with them, which depends on the account tier you're at: no infinite storage like with Youtube. (And with 4K video nowadays, that can be burned through quickly.)