Leader comment: Â£1bn bribe to stay in power will come at a heavy cost

It's a deal: Theresa May greets Arlene Foster and the DUP contingent at Downing Street.

It has been said frequently that this month's general election was unnecessary. There has been debate over this verdict, with some arguing that the ballot was in fact required, because Theresa May was, up to that point, an unelected Prime Minister. But what is harder to deny is that the election was the most expensive in British political history. It was expected the logistics alone would cost Â£140 million, but that figure has now been put in the shade by a deal between the Conservatives and the DUP. Having spent Â£140m from the public purse to achieve very little, it has now cost Mrs May a further Â£1 billion to re-secure the political power she relinquished by calling an election she was sure she would win.

There is a black-and-white way of looking at the deal struck with the DUP to give Northern Ireland £1bn of extra funding in exchange for votes: it’s nothing to do with Scotland, so Scotland is not entitled to a financial kickback. Northern Ireland won the election lottery. Scotland didn’t.

However, the deal highlights a grey area. The Barnett Formula was designed to ensure changes to funding in one part of the UK – in effect, England – are applied proportionately in the rest of the UK. In addition, its application can be discretionary. Funding can be allocated outside the Barnett arrangement.

Two factors leave the deal compromised here. First of all, the extra funding for Northern Ireland is to be spent on areas such as education and health, where Scotland and Wales have a decent claim on equivalent treatment for these devolved responsibilities. But more damagingly for the Conservatives, their Scottish Secretary has said he would oppose a deal that “deliberately sought to subvert the Barnett rules”. By implication, this suggests David Mundell believed any deal would by subject to Barnett. If the announced deal doesn’t subvert the Barnett rules, then it is hard to come up with an alternative, unacceptable, arrangement he had in mind.

Mr Mundell’s unease is symptomatic of the situation the Conservatives find themselves in. Mrs May’s authority is undermined, her leadership faltering, and her days looking numbered. Clinging on to power now comes at a price of £100,000 per vote in the Commons, and the only part of the UK where this move will be popular is Northern Ireland.  It will be no surprise if history judges Mrs May’s gamble to be good money thrown after bad.