An easy way to figure out the cost of powering devices is the Buck A Watt rule. In California, electricity charges of $0.15/kWh are common (some states have lower charges). If a device uses one watt of power, and you run it 24/7 for a year, the cost of powering the device is $1.31 for the year. In order to keep it simple and make it feasible to do calculations in your head, just think of it as:
one watt for one year costs one dollar.
This is particularly useful for energy vampires: those devices like cable TV boxes and chargers that are always plugged in and always consuming some power. If you plug it into a power meter (e.g. a Kill A Watt meter) and get its wattage, now you can easily tell how much it is costing you. If it is a lot you can put the device on a power strip, and turn it off sometimes to save some energy and money.
Good luck, hope you save a lot of bucks and watts!