Anything that makes heat is expensive to run, and a tumble dryer makes a lot of it. For households that dry every load by machine, it is often one of the largest single lines on the electricity bill, and one of the easiest to trim.
Why it costs so much
A conventional vented or condenser dryer pulls a couple of kilowatts and runs for an hour or more per load. Run the numbers through the cost calculator and a daily load adds up to a serious annual figure, often rivalling the fridge-freezer despite running a fraction of the hours. The heat is the expense.
Cheaper ways to dry
The cheapest dryer is a washing line or an airer, which cost nothing to run. Drying outdoors when the weather allows, or on an airer in a well-ventilated room, removes the cost entirely. Spinning clothes at a higher speed in the washing machine first wrings out more water, so whatever drying you do afterwards is shorter. Even half your loads air-dried roughly halves the dryer's bill.
If you must use the machine
Dry full loads rather than dribs and drabs, since the machine uses similar energy either way. Clean the lint filter every time, because a clogged filter makes the dryer work harder and longer. Use the moisture-sensor or eco programme if the machine has one, so it stops when the clothes are dry rather than running to a fixed timer.
The efficient option
If you dry a great deal and are replacing the machine anyway, a heat-pump dryer uses markedly less electricity than a conventional one, because it recycles its own warm air rather than heating fresh air and venting it. It costs more to buy and dries a little slower, but for a heavy user the running-cost saving is real over the life of the machine.