From the abstract of a recent Microsoft patent application:

A computer with scalable performance level components and selectable software and service options has a user interface that allows individual performance levels to be selected ... To support a pay-per-use business model, each selectable item may have a cost associated with it, allowing a user to pay for the services actually selected and that presumably correspond to the task or tasks being performed. An administrator may use a similar user interface to set performance levels for each computer in a network, allowing performance and cost to be set according to a user's requirements.

Acidus wrote:

... uhhhhh time sharing as prior art?

Oh, but you are neglecting this part:

All this is possible because the metering agents and specific elements of the security module 202 allow an underwriter in the supply chain to confidently supply a computer at little or no upfront cost to a user or business, aware that their investment is protected and that the scalable performance capabilities generate revenue commensurate with actual performance level settings and usage.

And, as Bill Joy recently explained to Malcolm Gladwell about his experiences on the time sharing system at the University of Michigan:

"The challenge was that they gave all the students an account with a fixed amount of money, so your time would run out. When you signed on, you would put in how long you wanted to spend on the computer. They gave you, like, an hour of time. That's all you'd get. But someone figured out that if you put in 'time equals' and then a letter, like t equals k, they wouldn't charge you," he said, laughing at the memory. "It was a bug in the software. You could put in t equals k and sit there forever."