Off is a relative term in electronics, because there is almost always something watching to see when it gets turned On. A normal USB device is ~500mA i believe, so thats about 2.5W, depending on your board, your load on the VSB can be a bit different but 1A would be about the most for a normal board, so thats about 5W.
5W on a computer that is plugged in 24/7/365 is 43.8kW over the course of the year, if you say its about 15 cents per kWh then the standby power costs you $6.57 over the course of the whole year, not a ton.