I pay about US$0.30 per kWh.
40 W * 24 (hours/day) * 365 (days/year) / 1000 (kW/W) * $0.30 > $100 per year
If you're in a hot climate with A/C, raise that further as you've got to remove that much heat from the room as well.
It was well worth it to purchase a Intel Xeon E3-1265L V2 to replace a power-hungry CPU in one of my infrastructure servers, as well as to replace the oversized and inefficient 1U power supply.