alt Hacker NewsBilling per used or not idle cpu cycle would be quite interesting. Number of cores would just effectively be your cost cap. Efficiency would be even more important. And if the provider over subscribes cores you just pay less. Actually that's probably why they don't do it...
Don't most big clouds not share cores between tenants? I have a vague feeling that around spectre/meltdown this was stopped. I wouldn't be surprised to be wrong, but if you're dedicating a core to a VM, you're not going to charge less for unused CPU that nobody else can use.