You'll also need to cut the power to power supplies if you want to save every watt. For example, my desktop computer (display et al. not included) takes 2.2 W sleeping, and 1.7 W powered off.
With 10 cents per kilowatt, 2.2 W costs 0.00022 whole units of money per hour. 10 hours of sleep would come to cost 0.803 whole units of money per year.
Formula: 2.2 W * (0.1 M/kWh / 1000) * 10 h * 365, where M is some currency of money.