If you fully drain and recharge your phone everyday, then over a year you would have to feed it about 2,000 watt hours, or 2 kWh. At an average price of 12 cents per kWh that means that your phone uses about one quarter's worth of electricity per year.
http://www.forbes.com/pictures/ekhf45eigjl/ipad-1-50-per-year-15/