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According to measurements from Lawrence Berkeley National Laboratory, the average cell phone draws 3.68 watts of power from the outlet while it's charging and 2.24 watts when charged.
Apple's (AAPL, Tech30) new iPhone 6 charges from 0% to 100% in 1 hour and 48 minutes, which is about 10.5 watt-hours of energy. Multiply that by 365 days in a year, and you get 3.83 kilowatt-hours. Multiply that by the average U.S. residential electricity price (12.29 cents per kWh), and you get 47 cents per year.