You can calculate this easily by multiplying your electricity rate with the dehumidifier's power consumption rating in kW (1000W = 1kW) or by using an online energy cost calculator.
For example, the Ionmax ION632 desiccant dehumidifier has a power consumption of 420kW (minimum) to 720 kW (maximum). This is equivalent to 0.42 - 0.72kW.
In Victoria we're paying 28c per kilowatt hour (kWh) for standard 'peak rate' electricity.
(Note that your rates may differ based on your state / territory, your energy provider, and your peak and off-peak rates. You can check your rates in your bill or on your energy provider's website. )
So to calculate how much it'll cost to run the ION632 per hour, we can multiply the consumption rating in kW with the rate per hour:
0.42kW x 28c = 11.76c per hour
0.72kW x 28c = 20.16c per hour
Therefore the running costs for the ION632 dehumidifier would be between 11c to 20c per hour depending on the mode / level that you're running it in.
If we use the same rate of 28c for our other dehumidifiers, then the estimated cost to run each dehumidifier is:
ION612 desiccant dehumidifier: 360- 620W
0.36kW x 28c = 10.08c per hour
0.62kW x 28c = 17.36c per hour
ION610 desiccant dehumidifier: 280 - 460W
0.28kW x 28c = 7.84c per hour
0.46kW x 28c = 12.88c per hour
ION681 compact dehumidifier: 100W (Updated to 75W in July 2017)
0.1kW x 28c = 2.8c per hour
0.075kW x 28c = 2.1c per hour