By Nick Robinson
BBC political editor
Four billion pounds (to be precise, Ł4.1bn). That is the cost of the Tory pledge to scrap the tax on savings for basic rate taxpayers and to increase tax-free allowances for pensioners.
Mr Cameron said savers are the 'innocent victims' of the economic downturn
So how will they pay for it? The answer is by spending less starting now.
David Cameron had already reversed his policy of matching Labour's planned spending increases for 2010 onwards.
Now he is saying he would spend even less this year too.
He has not, however, specified what programmes he would spend less on. That is not how government works. He has a point.
When any large organisation - the BBC for example - cuts spending its boss announces target savings and his underlings are tasked to identify how exactly they can be found.
That, however, has not been how politics has worked for the past two decades.
For three elections Labour have simply added up Tory "spending cuts" - in fact pledges to increase spending at a lower rate than the government - and then they have told voters how many doctors, nurses or policeman would go as a result.
In response, the Conservatives have specified savings in waste or government programmes that they cancelled.
In each case the Tories lost the argument and the election.
BCC (that's Before the Credit Crunch), David Cameron and George Osborne concluded that they could not win this battle and that the next election would be all about "it's society, stupid".
Monday's announcement confirms that they have been forced to return to the fray.
Their hope is that the changed circumstances, falling interest rates, rising debts and Labour's acceptance that spending cannot go on as it has, will make their policy an election winner now as it has not proved to be in the past.