I don't think the PC in the off state is where the power savings occurs.
Vampire drains are usually external peripheral devices like high end printers, hard drives, and other devices that may still consume a significant amount of power even when idle (and even when they go into their own "power-saving" mode.
Additionally, as Sammahmood describes above - you can go a lot further with these devices. As an example I have a Windows Home server NAS. It has an atom CPU and 3 2TB hard drives and runs 24x7. Most of that time I'm asleep and it is doing nothing but burning through energy. This is the kind of setup you might expect in a typical small business if you were working as a consultant. With these strips, you could easily schedule times to shut down the NAS on weekends and outside of regular working hours.
I mean, the cost savings is going to be be dependent on your situation in a case-by-case basis, but I can think of a lot of examples where significant power-savings could be achieved. If you're running a 110v server class device in a small data-closet and you're not using pro-class PDUs or racks - shutting down a server on a timed schedule is going to save you a lot of energy compared to having that device running 24x7x365. If you don't have remote access needs and you're operating on an 8 to 5 schedule, why have those kind of machines burning dinosaur bones when they're not needed?
Contributr 
