Enter the amount of time it is running for, and the starting and ending temperatures of the water, and it will tell you approximately how much energy it is using up to keep the hot tap running.
7°C is a good bet for starting temperature in winter in the UK, perhaps more like 15°C to 20°C in the summer; and usually the hot tap will produce water at about 60°C.
And, in terms of money. The current prices can be found on this USwitch Calculator. At the time of writing, electricity is 13.55p/KwH and gas is 4.162p/KwH on standard tariffs.
How much water used per minute when you leave the tap running?
Volumetric Heat Capacity of Water at 25°C
(This calculation approximates by using the heat capacity of water at 25°C rather than taking account of the change in heat capacity with temperature, but it is accurate to within less than 1%).