The International Greenroof and Greenwall Projects Database will provide you with information on completed green roof projects, along with the sq. footage of each installation. This database covers the years from 2002-present.
For average sq. ft. costs, see the US EPA web page on Green Roofs.
For examples of green roof projects see the American Society of Landscape Architects Stormwater Case Studies.
The Green Roof Industry Surveys will provide statistics for the last from 2004-2010. You will find much beyond that for a few reasons. Before 2005, no international green roofs database existed; the government does not compile statistics on green roofs; and green roofs were not in vogue 15 years ago.
For articles on green roofs and developing a market for them, search the Green Roofs for Healthy Cities (GRHC) Tree of Knowledge database. GHRC is the primary organization that represents the green roof industry in North America.