Well, I never did any calculations. But i've read in other places it can cost anywhere from 10-20$ a month running 24/7. Depending on hardware, how much your stressing the hardware, how long your using it, how large the LCD screen is, etc.
IMO ~15$ a month sounds about right. Depending on where you live of course, and the cost of power in your area.
Don't forget that what hours of the day it is running matter as well. Electrical companies in some areas charge double rates during what they consider "peak hours". So having it off while you are out of the house in the middle of the day and running at night while you are sleeping will run you different charges than running during the day and off at night. If you are wanting to do something like running F@H then running it at night will actually save you a little bit.