It would be in the 45w-70w range depending on your power supply. I had for a while a machine with an e8400, a cheap Acer m-atx motherboard with igp and a couple of ddr2 slot both filled with an 1GB single rank ddr2-800, an intel pro/1000 pt dual and finally an v300 ssd. It pulled 54w while idling. I did some math, in my country energy cost roughly 0.2€/Kwh so each watt cost 2€ per year after some calculation. I did a quite complex excel file to take into consideration every variables, so I'm pretty sure. You can expect almost 110€ per year on electrical bill or 9€ per month if you prefer. Of course, your mileage may vary a lot depending how the electrical bill is calculated then the cost of the energy per Kwh, I bet it would cost less but not enough to don't take into consideration to buy brand new low power parts as I just did. Replacing the cpu+mobo+ram combo and then the psu with an old 500w+ 80plus I already had it dropped the power consumption at just 20w. At worst. I can go further with a pico psu or a brand new psu, but right now I'm quite satisfied, I save more than 70€ per year whit much more power available.
Ciao.