Much less than Petrol or Diesel! At home, it's your standard electricity rate, which is probably in the region of around 14p per kW (the UK average is 14.37p). So, a car with a 20kWh battery will cost less than £3 to fully charge. This cost can be almost halved for people with an economy 7 meter, charging at night on cheaper rates. Out and about, public charging facilities have costs which vary between networks and charger type - they can range from completely free to around £6 for half an hour on the fastest "Rapid" chargers.
However an electric car is being charged, without much effort it's possible to achieve around 5p per mile. With a bit more consideration around how and where you charge, it can be under 2p per mile. What's more, if you have your own solar panels at home, you can charge from the sun - for nothing.