How much does 1 Space Shuttle external tank cost?

According to a report on the SpaceRef web site linked below as 'Lockheed Martin Awarded External Tank Contract', NASA has a contract with Lockheed Martin, New Orleans, that totals $2.94 billion to build 17 external tanks through Sept. 2010. That comes out to be an average price of around $170 million per tank.

However, the report mentioned above is about an addition to the external tank contract, so it may be a misprint and the 17 external tanks may be additional tanks added to take the contact to the end of the shuttle program. According to the reference linked below as 'The Assembly and Production of the External Tank', each tank costs about $55 million as of 1995.

Later tanks, called the Super Light-Weight Tanks (SLWT) cost around $5 million more than the earlier version, the Light-Weight Tank. The SLWT costs more because they use an aluminum/lithium alloy that is more expensive and harder to work than earlier materials. By making the tank lighter, the shuttle is able to carry a heavier payload. This change allowed the shuttle to meet the payload requirements for construction of the International Space Station.