Well, the cost is so variable it's hard to come up with. You have to take your cost per KWH into account, the condition the dryer is in, how clean is the lint filter. How effective is the duct to the outdoors, etc. Here is a method to use to calculate all this.
First call the electric company and find our what rate you are paying per KWH.
Wash your towels.
Then go to your circuit breaker box and kill all the circuits except for the dryer.
Take a reading at the electric meter.
Dry the towels
Reread the meter and do the math. This will be your approximate cost.
Or I think there is a device you can plug the dryer into that will tell you how much has been used. But I am not familiar with those, I think Tolivac or Toggles can offer more info about these.