where are you getting the £2.5 billion figure from, whitewave? It's generally being quoted as £1.5 billion.
The costing the tories estimated for Labour's manifesto pledges adds up to £2.2 billion.This would have come from the same magic money tree that May has suddenly discovered but, unlike May's bribe to the DUP, would have benefited the whole country and been a source of revenue in profits from renationalised industries and taxes on increased business activity and increased wages.
As for 'borrowing' it you really should get yourself up to speed with Modern Monetarist Theory roses. Governments don't 'borrow' money, they create it. So far governments have 'created' some £345 billion in the last 10 years for quantitative easing to bail out the banks and to keep the economy stable after the Brexit vote.
I don't exactly grudge NI the money, they need it, but to get it in the form of a bribe to keep a tottering government in office is an abuse of power. Not only is the DUP currently embroiled in a scandal over £millions of missing money, raising questions about whether the 'new' money will be used as it is supposed to be used, but this bribe has serious implications for Power Sharing (on which there is still no agreement in Stormont) and the Good Friday Agreement.