|If you were to translate the savings into $$, I don't think you'd save more than a few cents a month. The real savings is in runtime while operating on the battery. By powering down the hard drive, monitor, etc., while they're not in use, your runtime on a single battery charge can be extended by as much as several hours as compared to leaving them run all the time.|
Most systems these days can also "step-down" the processor speed while on battery power. This will reduce the amount of heat produced, but again, the real benefit is that you'll be able to run longer on a single battery charge.
Most if not all of the available power settings can be accessed via "Power Options" in the Windows Control Panel. The manufacturer of your system may have also included their own utility to change these and other more advanced settings.