The Sydney Opera House ultimately cost about 14.6 times the original estimate. The 1957 estimate was £3.5 million, commonly expressed at the time as about A$7 million, while the completed project cost approximately A$102 million.
The increase was driven by the extraordinary difficulty of turning Jørn Utzon's competition concept into a buildable structure. The design used unprecedented concrete roof forms, and engineers had to resolve major structural problems while construction was already under way. The government also changed the brief during the project, increasing the complexity of the interiors and performance facilities.
Work began in 1959 before all the final designs were complete. The building was intended to open in 1963, but it was formally completed in 1973. By October 1966, the project had already cost A$22.9 million, yet the final stages—including extensive interior work after Utzon's departure—pushed the total above A$100 million.
The wording can cause a small mathematical trap: A$102 million divided by A$7 million is about 14.6. Strictly speaking, that means the final cost was about 13.6 times more than the estimate, or 14.6 times as large. Trivia references usually use the latter convention.