It depends on the air conditioner. They come in many different sizes. It should have a label that tells how much the maximum power consumption is. Example: 2.4-kW (120 Volts, 20 amps)
You can work out the cost by taking the power consumption and multiply it by the cost of electricity time the number of hours of use. For example: 1.5kW x $0.15 kW/h x 8 hours = $1.80
It depend with size and it's usage. In size. Larger units usually have larger energy needed. It also matters in times you are using the unit, it also cause high cost.