(1) You need to measure how much electricity (watts) your miner is using at the wall. Best way is to use a Kill-A-Watt meter. This is your WATTS.
http://www.amazon.com/P3-International-P4400-Electricity-Monitor/dp/B00009MDBU

(2) Figure out how many HOURS your miner is running. This is your HOURS.

(3) Multiply WATTS x HOURS / 1000 = KWh

(4) Find out how much your electric company charges you (per KWh). Multiply by above and you get your cost.

(5) You can use this calculator to help as well:
http://bitclockers.com/miningcalculator