I did some googling myself and found this relatively comprehensive answer:
A totally negligible cost. Mind you that the calculation linked above assumes that one recharges the battery for 12+ hours. These things are also called “trickle chargers” for good reason. If the person only plugged it in for 1 or 2 hours while washing the car the charges would be even more ridiculously low.
In that case, if one wanted to play devil’s advocate, once could even argue that a vacuum cleaner uses many times more power than this trickle charger. Probably even the water used for a car wash costs more than what charging the battery cost. Then you get into territory where you have to start asking: Should we bill people for the water (install a water meter) and for the electricity for the vaccum? What about people who go to the car wash? Why are they punished by other people wasting water and electricty. It’s a can of worms but armed with the calculations provided here you should be able to avoid opening it in the first place.