My electricity cost is very cheap, I have 1 regular computer running 24/7 and one laptop running 24/7. On top of that I have 5x ASIC 333 mh/s memories mining constantly, and they are in a USB-hub, I rarely turn off the lights but my electricity bill lands around $40 a month so that wont be a problem.
Computers use very little power these days. How much do you pay per KWH. Check your bill. If its 10kwh or higher then forget about mining.
Do you mean how much I use? 10KwH what? I pay ~$0.3 per kWh right now, last month I used 207 kWh. I know computers are cheap but I always have all my lamps on unless I sleep so that should also count into the energy usage.