I used to keep my computer on 24/7 to save wear and tear caused by turning it on/off all the time. Then few years ago I did the math: with current electricity prices this is ridiculous. Did some more research and easily cut my electricity bill to 1/2. And that’s a nice amount of €€/month.
At my location it’s easy to calculate approximate cost: 10W = 1€/month. (OK … actually it’s more like 1.15€)
And don’t forget to unplug the cell phone charger etc … by unplugging all these little gadgets in you home you may even save $2/month, if you’re lucky 