The Great Wall of China at Badaling is around 90 minutes from the Beijing center which would be the Tiananmen Square area. The China travel maps that they sell on the street in Beijing make it seem that you can take a city bus, but that is not the case. They put you on special tour busses that take you to the Great Wall. You find the tour busses south of Tiananmen at the Qiamen half circle which is one very long block south from Mao's Mausoleum. The bus ticket office is on the North West corner directly across the street from the Qiamen Stop for the Beijing Blue train you will know that you are in the right area as there are 50 bus stops on the other side of the street. It is important to remember that these are white busses and the city busses are multicolored.
The price of the bus depends on the tour that you take ranging from 160 to 90 Yuan for a shorter trip. They have two trips to the Badaling section of the Great Wall that leave from between 6:30 and 8:30 AM expect to get back after 5 probably closer to 6 if you are on the early bus. The thing to remember here is that they want you on the bus regardless of what your circumstances are, they don't care if you miss a cheap flight or other travel connection because they gave you bad information so make sure that you question them thoroughly if you are on a schedule.
The longer trips on the bus include a lunch and the fees. We were told that the fees were 90 Yuan this price it turns out was for the two of us. The entrance fee for the Badaling wall at Beijing was 45 Yuan.