The power usage varies widely on a system, hard drive is one of the big pulls in a system, so if your hard drive isn't spinning, that is less power used. The best way to find out would be to hook up a meter to your system and measure. A fluke set on current inline with your power cord should be able to give you the total output.
Now, according to a study funded by "Thin Client Computing" (an organization that wants to take "pc's" out of the office environment and go back to terminal type clients with a central server for software and data access) A PC running uses betwwen 65 to 85 watts of power. Approximately mid 60s at idle with no applications or hard drive access, at the high end of the scale when fully in use. These numbers were collected from a 1.5 GHZ P4. The numbers for other systems are comparable with the avg difference being as little as 5 watts for as much as a 1 ghz difference. All bets are off if the CPU is OCed. This significantly increases the heat rate of the processor and completely throws these numbers out the window.
Here in VA, the rates are as high as 6.05 cents per kwh to as low as 3.20 cents per kwh. These rates vary based on time of year and total power usage by the customer. In the summer with high power usage the 6.05 rate applies, however in the winter with high power usage the 3.2 cent rate applies.
So, .1kwh * 24 hours = 2.4kwh per day
2.4 kwh * 30 days(avg length of months) = 72 kwhs per month.
figure that times the high end of the VA scale 72kwh * 6.05 cents =$4.36 / month
At the low end of the scale 72kwh * 3.20 cents = $2.30 / month
Overall the cost of the box itself running is negligible.
Sorry if it sounds like I am preaching, but wanted everyone to know where I got the info from. It makes it more understandable imho.
Cliff Notes.
Box = $2.30 to $4.36 per month.