The cost of moving the Powerhouse Museum to Parramatta could blow out to almost $2 billion – 10 times the estimated sale price of the museum's Ultimo site.

Secrecy surrounds the NSW government's controversial plan for the Powerhouse, but a parliamentary inquiry was told last week that construction of a new museum would cost up to $800 million.

Lindsay Sharpe, a former director of the Powerhouse from 1978 to 1988, later told Fairfax Media: "What can safely be said is that there will probably not be much change left from $1.5 billion and it might go as high as $2 billion, in 2024 dollars."

The cost of relocating the Powerhouse also involves the purchase of land on the Parramatta River, which has been valued at $150 million, and moving the museum's vast collection, including its priceless Boulton and Watt steam engine.