The British Conservative Party reportedly paid stg£20,000 (€22,798) to fly DUP leader Arlene Foster back to Belfast in an RAF plane after Downing Street talks - despite commercial flights being available for as little as stg£41 (€46).
According to the Mail on Sunday, the RAF flight took place after Ms Foster agreed a deal with British Prime Minister Theresa May to support her minority government.
The Conservative Party has been forced to foot the bill for the trip after being told the British taxpayer cannot pick up the tab, the newspaper also reported.
A Conservative spokesman confirmed the party had paid for the flight, but did not say how much it cost.
They said: "Arlene Foster was in London to conclude the signing of the confidence and supply agreement between the Conservative Party and the Democratic Unionist Party which will provide the UK with the stable government it needs at this critical time.
"The Conservative Party paid for a flight back to Belfast to allow her swiftly to resume talks on re-establishing a power-sharing executive in Northern Ireland."
As part of the Tory-DUP deal, stg£1bn (€1.13bn) will be made available to spend in Northern Ireland, but British Environment Secretary Michael Gove has insisted the cash was not a "bung".
Mr Gove said the agreement was needed to ensure a "secure and stable" government - and highlighted Northern Ireland's unique problems, like dealing with the Troubles and needing investment in mental health and infrastructure.
Mr Gove told the BBC's Andrew Marr show: "I think that's unfair (to say it was a bung), I think that's unfair to the people of Northern Ireland and I think it's also unfair to the way in which, during this negotiation, decisions were taken in the interests of everyone in the United Kingdom.
"Bung is, the implication is... it implies this money is somehow going to the DUP on their own as if it were a partisan deal. It's not.
"It's about strengthening the whole United Kingdom by helping people in one of its most vulnerable areas."
As part of the deal the Tories have scrapped manifesto pledges to end the pensions triple-lock and to means-test the winter fuel allowance for pensioners.
"It's absolutely right that we should, after a general election in which we didn't secure a majority, that we should have an opportunity to review how we help the most vulnerable in our society," Mr Gove said.
