Santos said in a statement the $US380 million figure assumed a future oil price of $US90 a barrel.
The oil and gas producer will supply about 33 petajoules of gas over six years, from mid-2010, for Moly's Spinifex Ridge molybdenum and copper project in Western Australia's Pilbara region.
The gas will be supplied from the John Brookes gas field and processed at the nearby East Spar gas processing plant on Varanus Island, offshore Karratha in WA.
It will transported to the project, which is near the town of Marble Bar, through the Dampier to Bunbury pipeline.
The John Brookes joint venture is 55 per cent held and operated by Apache Energy Ltd, the subject of intense criticism in WA after a pipeline explosion at the Varanus Island gas processing plant in June that cut one-third of the state's gas supply.
The John Brookes gas field and East Spar processing plant resumed production in August and Apache's other Varanus Island joint venture, Harriet, is expected to return to production in December.
Santos said its ability to deliver on the Moly Mines contract was driven by a net investment of about $45 million during 2008 and 2009 to increase the gas pipeline capacity from the East Spar facilities.
"Santos has again demonstrated a willingness to invest significant capital to ensure that the expanding energy demands of the Western Australia domestic gas market are met," Santos chief executive David Knox said in the statement.
The gas price achieved in the contract is linked to international traded crude oil price.
The contract is subject to conditions including Moly achieving its capital raising objectives.
Despite difficult financial markets, Moly Mines last month completed $US150 million ($A218.77 million) in debt funding for the $1.08 billion Spinifex Ridge project.
At the time, chief executive Derek Fisher said this interim financing would allow the company to continue development and construction of the project while it completed full funding arrangements.
