I pay about .09c per kWh for service and about .10c for delivery. Just over .19c per kWh total. Standard coned rate in NYC. Our power comes from the Indian Point Nuke plant, coal, and Niagara falls. (among others). I pull 275watts to run a 6870 so about $30 a month.
You must have some room for improvement where PC energy efficiency is concerned.
I have the same card as you, and my entire system -- everything plugged into the wall, including the wireless dish (my Internet provider) and router, draws 272 watts (not counting monitor -- though my large 24" LCD monitor only draws 20 watts when it's on)
Oh, and that's with CPU mining on all 4 cores.
I have a Quad Core Sandy Bridge (3.4 GHz -- stock speed, stock cooler). 1 TB Western Digital "green" hard drive, 6 GB DDR3 RAM, and no cards plugged in besides my video card (I'm getting a 2nd card today).
I have a 750W Corsair "80 plus silver" power supply -- that helps. 80 plus gold would be even better for efficiency (but the PSU would be more expensive).
I just took out my Sound Blaster X-Fi card yesterday, which seems to have saved 6 watts. My new PC has a pretty decent on-board sound card. I also disconnected the bright blue LEDs for my Power On and HDD indicator -- they were annoying when trying to sleep at night, plus they probably draw 1 watt between the two of them -- or maybe 1/2 watt. At any rate, they weren't doing anything for me.
add to that $9 monthly fee. 
