Thats a bit difficult when not knowing the price of electricy where you live, the power consumption of your computer and how long time you keep it on. However, it quite easy to calculate yourselves:
PricePrMonth [$/month]= PowerConsumption [W] * OnHrsPrDay [hrs/day] * 30 [day/month] * 0.001 [kW/W] * PricePrkWhr [$/kWhr]
E.g. 300W, PC on 24 hrs pr. day and 0.2$/kWhr:
PricePrMonth = 300*24*30*0.001*0.2 = 43$
<i><b>Engineering is the fine art of making what you want from things you can get</b></i>