I could get you some actual numbers buy measuring the watt draw from the wall on a completely dead battery... But I am also far too lazy to do that. But in Seattle at somewhere around 10.2-10.8 cents per kilowatt hour, that would probably equal around 6 to 7 cents per charge.
A 48v battery with 11.6ah capacity has about 560WH. Electricity with my supplier is 10cents per KWH, but considering how much the adapter heats up(very inefficient), you could probably double the wattage needed to charge the battery. So lets say 1000WH to charge the battery. That's about 10cents per charge of a fully dead pack.
To know how the true wattage necessary to charge, you would need to log it with a kill-a-watt meter.