Those costs would come on top of the $30 million per month the government is already paying to warehouse the New Intercity Fleet (NIF).
"Let me be very clear. This government does not want to flush any money down the drain," state treasurer Matt Kean told media on Friday.
"We treat taxpayers with respect. So we want to make sure that every cent that taxpayers pay is used wisely.
"There's $2 billion worth of rolling stock sitting in mothballs behind me rusting on the tracks because of these unreasonable demands from the union."
The Rail, Tram and Bus Union say the NIF is a "disaster waiting to happen" due to poor guard visibility, and their assessment is backed by an independent review and the Federal Court.
On Thursday, Finance Minister Damien Tudehope announced he had cancelled all talks with the transport union.
The move shattered a recently brokered peace deal and five weeks of intensive negotiations led by Transport Minister David Elliott, after months of disharmony and a government-led shutdown of Sydney's rail network earlier this year.
On Friday Mr Tudehope said the union had not gone through appropriate processes with the regulator to raise its safety concerns over the NIF, insisting the trains were safe.
"Now if there are those concerns I encourage the union to raise them with the regulator," he said.
A letter sent on Thursday from the Transport for NSW Deputy Secretary Matt Fuller to the union with designs for modifications to the fleet had been misinterpreted as a commitment to upgrades, Mr Tudehope said.
"It wasn't an undertaking to do that work," he said.
"It was against a background - if we were to do that work, could it be done?"
Mr Tudehope said he would not submit the claims to the expenditure review committee to alter the trains, calling them the "most advanced technology comparable to anything in the world".
The billion dollar figure was put together by Transport for NSW, and suggested upgrading costs included $385 million in direct costs, approximately $420 million in delay costs and $27 million in additional planning costs.
Mr Elliott said he is waiting for an audit of the billion dollar figure, but he intends to prosecute the union's request to the government's ERC.
"In my good faith arrangements with the union I said, 'I can't guarantee that ERC will agree to all of your requests'," Mr Elliott said Friday.
"But I have given them a good faith commitment that I will prosecute their claims."
RTBU Secretary Alex Claassens said the claims from the finance minister showed a disregard for the safety of commuters and misrepresented the findings of the regulator.
"These trains are a disaster waiting to happen and there is an independent safety assessment, among other things, that confirms that," Mr Claassens said Friday.
"Rail workers will not drive a train that we know will put the safety of the travelling public at risk."
Opposition Leader Chris Minns said any claims the treasurer made about the costs to upgrading the NIF needed to be substantiated.
"One of the big problems here is that these trains should have been built in Australia, made by Australians," Mr Minns said, adding that the fleet was constructed in Korea.
"These trains are already two years late and $1 billion over budget."
"Under the premier's predecessor, Gladys Berejiklian, we had a decade's worth of industrial peace, because common sense and maturity when it comes to these sorts of negotiations took place," he said.
"That is not happening under the premiership of Mr Perrottet."
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