A photo of Britney Spears buying coffee sold for $250. A photo of her crying sold for $100,000. That 400x gap explains this whole video. Every photographer on that sidewalk understood that a calm walk to the car earned them lunch money while a mother stumbling with her baby paid a month's rent, so the smart play was to press in until something went wrong. X17 alone kept a crew on her around the clock and moved $2.5 million worth of Britney photos in 2007. The head-shaving pictures went for…