Immersion heater is 3KW, so whatever you pay for your leccy per KWH (kilo watt hour) times by three per hour of use.
e.g. 20p/KWH = 60p per hour to run & you'll need it on for 4 hours with a 400ltr flat tank. Mine's on a 16amp timer with off peak leccy & costs approx £1:60 per day to power. Water holds it's temperature in the wydale plastic tank all day long.
Also IMPORTANT- you'll need it rigged up into a 16 AMP breaker in main consumer unit, use a 16amp extension cable with 16amp plugs. Learn from my mistakes & don't risk burning your house down!!
The major benefit of the immersion route is the fact that the tank acts like a giant radiator & so nothing in the van freezes.