Question: how much does it cost to run a boiling water tap?
Answer: Boiling water taps and hot water taps are a premium purchase, so there's quite a lot of initial outlay. However, Quooker (whose Fusion model is shown below) says that their boiling water taps can cost 3p per day to run on standby. A conventional kettle costs, on average, about 2.5p per boil.