well, I use a kilowatt or any other watt meter. It isn't exact, but it can tell you about what your computer is using for power in Watts. You take that numbe, lets call it x, and:
x*24/1000*(local power rate/KWh). That will give you the cost if it is running for 24 hours. the 1000 is a conversion from Watts to KiloWatts.
for example I have a quad G34(6174) system that uses about 500W. My local power rate is $0.11/KWH
500W*24/1000*0.11=$1.32 for every 24 hours of running at 100%.
If you are just looking for an estimate it would be helpful to know the system specs. (I'm guessing it's the one in your signature) if so I'd estimate the power usage at around 250275W. Might be a little high but it's a place to start.
