To fill the tub requires roughly 100 litres of hot water and 100 litres of cold water. In Reykjavík, the cost of 100 litres of hot water is around 14 ISK.
Sometimes it is said that hot water can be used much better if we take a shower instead of filling the bath tub. A closer look reveals that the difference is around 30 litres of hot water or 3-4 ISK every time.
Taking a bath is an inexpensive luxury that surely can be recommended!