(CNN) -- A prolonged undersea search for the missing Malaysia Airlines Flight 370 could cost nearly a quarter of a billion U.S. dollars if private companies are used, Australia's top transport official said Thursday.
Martin Dolan emphasized that the $234 million price tag is a "ballpark rough estimate" of an extended search and salvage mission that includes an underwater vehicle.
The Bluefin-21 is back at work Friday morning on a fifth trip into the southern Indian Ocean. Authorities said the vessel has scanned a total of 110 square kilometers (42.5 square miles) without making any "contacts of interest."
Searchers seem to be preparing for the possibility that an underwater drone scan of the ocean may not yield debris from the plane immediately.
Malaysia's acting transport minister, Hishammuddin Hussein, said that authorities are looking at deploying more unmanned underwater probes.
Officials might consider searching along a large portion of sea highlighted by a partial digital "handshake" between the jetliner and an Inmarsat PLC satellite, Dolan said.
That arc of sea is over 370 miles long and 30 miles wide.
Officials didn't indicate whether Bluefin's fourth mission was a lengthy one.
The Bluefin was forced to abort its mission twice this week; the first time after it exceeded its original depth limit and the second time over a technical issue. After the latter was resolved, officials dipped it into the ocean again.
Phoenix International Holdings, which owns and operates the equipment under a contract for the U.S. Navy, called the third dive "very productive," saying that the underwater drone spent nearly 13 hours at the sea bottom.
But after four dives, "there has been no debris or aircraft wreckage discovered," the company said.
The underwater vessel takes two hours to get near the ocean floor and another two hours to return to the surface. It aims to map the ocean floor for 16 hours to retrieve data, which then take four hours to analyze.
The vessel searches maximum depths of 4,500 meters (14,764 feet). The U.S. Navy has determined the seafloor in the search area reaches a maximum depth of 4,600 meters (15,092 feet).
Bluefin operators said they can reprogram it to operate at 5,000 meters (16,404 feet), giving it more leeway.
Officials believe a recent ping heard in the search area has the right frequency to belong to the flight data recorder's emergency beacon.
The quality of the "ping" led authorities to focus the underwater search in the area.
And as the underwater focus continues, officials faced other setbacks.
A preliminary analysis of an oil sample collected in the ocean shows that it is not aircraft engine oil or hydraulic fluid, Australia's Joint Agency Coordination Centre said Thursday.
Malaysia Airlines Flight 370 vanished on March 8 with 239 people aboard after taking off from Kuala Lumpur, Malaysia, bound for Beijing.
With no debris found so far and no possible pings from the plane's "black boxes" detected in a week, officials shifted the focus of the search underwater.
CNN's Ivan Watson, Brian Todd, Elizabeth Joseph, Erin Burnett and Steve Almasy contributed to this report.