that is too hard of a question to answer, due to the varying price of electricity and with everyone having different electrical usage within thier home (extra fridge, a/c versus fans, etc).
but if you wanted to calculate how much energy a game uses, you can use the following calculation: 9(Watts= Amps x Volts)*30days )/1000 will provide you the kWh usage for 1 hour a day for 30 days.
example.. if a game runs at 2amps and 110 volts, the wattage is 220 * 30 days=6600/1000watts = 6.6 kWh's for the month if i ran the game 1 hour a day all month. at about $0.15 per kWh here, that is about a dollar..