If you ran a computer 8 hours a day and it used 500 watts (average per hour) it would cost about $182.625.
(Which is 50 cents per day, and this is a worst case scenario for most machines).
This assumes about 1461 kilowatt hours per year at 12.5 cents per kilowatt hour.
Most modern computers only use about 180 to 360 watts under heavy compute loads, so expect it to cost less, even if your electricity does cost +25% more than ours.
Note: In Western Australia energy is 13.94c per kW/hour.