Between 1984 and 1993, $11.2 billion was spent on Space Station Freedom, the U.S. predecessor to the International Space Station (ISS). When the ISS replaced Space Station Freedom, President Clinton pegged the cost of the new project at $17.4 billion. In 2006, the Government Accountability Office said that $35 billion had been spent since 1985 and that it would take about $100 billion to complete the station [source: Boyle].
There are many complicating factors in figuring out the station's approximate cost. It receives major contributions from a group of countries: the United States, Russia, Japan and 10 members of the European Space Agency. Shuttle flights, refueling, maintenance and other related costs may or may not be included in estimates, likely contributing to NASA saying that the station will be cheaper than the GAO's estimate. There are also unforeseen costs and setbacks, such as the tragedy of the Space Shuttle Columbia disaster.
When considering costs, are we asking how much is it to complete the station or to complete and run it for a certain period of time? The European Space Agency states the estimated cost will be 100 billion euros ($123 billion) to assemble the station and run it for 10 years [source: ESA].
No matter the final cost, the space station is certainly the most expensive science experiment ever attempted. But proponents of the station argue that it is an unprecedented opportunity for international cooperation, to test the limits of space technology and to conduct valuable, potentially groundbreaking research in a unique environment.