According to David Foster, it is the cost of 'free' water : 
What I'm attempting to convey metaphorically through these images is the cost of 'free' water supplied by tankers or a public standpost. The time taken by the women (and its almost always women or young girls) to reach the source, wait for the water, and go back home could be used more productively if the households had water connections. Although policy makers recognize the "opportunity cost" when their own time is wasted, we often forget that the poor experience "opportunity costs" as well. We can try to estimate the value of that lost time by thinking of other household chores, handicraft making and educational opportunities that are forever lost because of the time demands for carrying water. Although water supply in virtually all Indian cities is heavily subsidized, very few BPL (Below Poverty Line) families have household connections. While upper income families benefit from those subsidies, the BPL families are often forced to buy water from private vendors or carry water from public stand posts, water tankers, or public fountains and wells. Unfortunately, even where this water is provided free of charge, we often underestimate the true cost incurred by the poor in obtaining it. Spend a few minutes with the interactive spreadsheet provided below and see what you think is the real cost of that "Free"water provided to the poor. Not only will this exercise help us to better understand those costs, but it will also help to emphasize that any planning for the poor must always be done with the poor. Once planners and citizens better understand the real size of these costs, they will also understand why household connections are usually a much better (and cheaper) way to provide water than "free"public stand posts.
Download spreadsheet : This interactive spreadsheet and the discussion regarding the real cost of "free"water was provided by David Foster, a water policy specialist working with the Administrative Staff College of India, in Hyderabad. Anyone interested in further discussion on this topic is encouraged to contact David at Dafoster@aol.com .