Why do you suggest punishing people who are willing to use the infrastructure with a premium percentage? Reminds me of EC2, absolutely sucks as you have to pick the right hardware, the right reserved instance, etc.
Digital Ocean does it at the same price as the monthly price. It's simple. I know I'm getting the same cheap price regardless of when I decide to spin up an instance, or when I decide to kill it.
Personally I'd like to see Linode do a billing system similar to
Google Compute Engine - minimum 10 minutes and pay by the minute. Keep it simple, same pricing structure as the monthly plans.