There are a couple of methods for a Twizy or a car using a EVSE brick. Use the @ecofunkytravel suggested Plug in Power meter.
Or for a more generic route for all cars.
Take the usable battery size in KWh and multiply by the cost of Electric. Allow between 5 and 10% for losses.
So a Twizy battery Say 8KWh (allowing 10% for losses) times peak rate 15p = £1.20 for a absolutely full charge. Usually the battery isn't empty.
or a Leaf 24KWh battery (only 22KWh usable) but 25*.15 = £3.60
So it pays to use cheap rate electric and or Solar. Twizy comes out at 40p on E7
All depends on the rate you pay for Electricity.
@james mentions Tapering. Tapering is where the rate that the battery can absorb the charge drops off. ie. a battery on a Leaf may charge up to 80% in 30 minutes using a lot of current (watts) and then draw less power for then next 2 hours. However the overall power draw will still be the same 24KWh. 19.2KWh going in fast and the last 5 goes in slowly. Still only 24KWh.