If you need to find dishonesty in politics, look no further than the phrase "fully costed".

It's the worst kind of lie: sounds precise, careful, reliable - with real numbers, too - and means nothing at all. Choices yet unmade cannot be costed. The implications of one future choice for other choices can only be guessed at. Politicians know this.

They also know that plucking figures from thin air is a bad habit, often punished. Such is the purpose of "fully costed". It cloaks naked bribes and shabby electioneering in the semblance of respectability. If all you can do is throw big numbers at the voters without offering a few sums on the back of an envelope - show your working, as they used to say - you will look and sound desperate.

George Osborne, the Chancellor who once claimed to have all the answers, looked the picture of desperation on Sunday's Andrew Marr Show. It should have been simple. The man who does the books for UK plc, the one who has spent five years lecturing us on the condition of the national finances, should have been able to say instantly where he means to find an extra above-inflation £8 billion a year for the NHS by 2020.

Opinions differ only over the number of times he failed to answer. Fifteen times? Eighteen? Viewers were left to guess: was the Tory pledge costed, or is eight just one of Mr Osborne's lucky Lottery numbers? If so, one of his other numbers is 12, as in the £12bn in benefits cuts he seeks. Once again, he cannot or will not tell us - with lives hanging on the answer - where he would find that money.

It isn't saying much to say that the Tory election campaign has become shameless. In this season, pledges are chaff in the wind. But to see Mr Osborne and his colleagues turn their austerity rhetoric into confetti without a blush is something else again. Where there was a desert with "no money", the cash gushes. Where once five years remained to finish that legendary "long-term economic plan", suddenly the UK is back at the races and the Tories are spending the winnings.

There's a discrepancy that bears a close resemblance to a lie. Either austerity has been a fool's errand using debt and deficit as an excuse to "shrink the state", or the election pledges are hokum. Possibly both. Either Conservatives have been satisfying an old contempt for the benefits system and those who need it, or the largesse is founded - by Mr Osborne's own arguments - on fiction. Set £12bn to be carved from benefits against the promise of an inheritance tax cut for the better off and judge this chancellor.

His Budget projections were based on the claim that fiscal tightening would continue until at least 2018-19. Now he and David Cameron promise to "freeze" rail fares in real terms for five years. Of itself, the policy has merit. Travellers have paid a heavy price for the nonsense of rail privatisation. But coming from the party that has insisted on stringency in all things, this is transparent hypocrisy. Labour, for one, prices the rail offer at £1.8bn. So is "the job done", as Mr Osborne likes to doubt, or not?

We'll get our answer after May 7. Whether the NHS also gets that £8bn might be another matter. Mr Cameron launched his manifesto yesterday with the air of a man who had just discovered the milk and honey river. All politicians, you might say, are at it this April, but the Prime Minister who told us that austerity is unavoidable was ladling out his treats as though all previous statements had become void.

Mr Cameron, Cotswolds representative of "the party of working people", says he will legislate to keep those on the minimum wage out of tax, impose the "right to buy" on housing associations in England, double the amount of free childcare available to parents, and still be running a surplus by the end of the next parliament. I make that one and a half good ideas in four. I don't notice one bearing the label "cost-free", however. The party that aims to win the election on claims of economic rigour is vague about figures.

Minimum wage earners out of tax? The reform would be handier for higher earners, in addition to an existing Tory pledge to raise the 40 per cent threshold. Yesterday, Mr Cameron's team admitted as much. What they did not confess was that a reform restricted to those working 30 hours a week is not much of a giveaway. National insurance, tax by any other name, remains.

In any case, 30 hours will not get you above the existing £10,600 personal allowance even after the minimum wage goes up to £6.70 in October. If Mr Cameron is being truthful about keeping such earners out of tax in future, he either means to sacrifice billions, or pull off a sleight of hand, staging increases in the allowance and keeping the cost to a minimum. The issues of low pay, poverty traps and a lack of hours are not addressed.

Tax cuts generally are another matter. Helping those who regard inheritance tax as a problem will cost £1bn, at least. Raising the personal allowance to £12,500 over the course of a parliament, as Mr Cameron promises, will cost a minimum of £8bn according to the Institute for Public Policy Research. Simply raising the higher rate threshold to £43,300 by 2017-18, as Mr Osborne has previously announced, will set the Treasury back £2.5bn.

You can argue the merits of these proposals. What you cannot do is pass them off as the hair-shirt economics that has been the Chancellor's stock-in-trade for five years. Mr Cameron behaved yesterday as though the era of austerity is over. In another mood, Mr Osborne has a deficit approaching £90bn in a country with debts of £1.5 trillion that say otherwise. Which is it? Or was austerity nothing more than an opportunity for ideologues?

Perhaps a housing boom would help. The right-to-buy scheme for England is the kind of opportunism Margaret Thatcher would have enjoyed. Sell off valuable stock, use the proceeds to subsidise the sales, build affordable housing, fund brownfield regeneration, and call the entire shanty town of cards "fully funded"?

Mr Cameron says he will wring an improbable £4.5bn from housing association stock and thereby address the housing crisis. It is more likely, if not inevitable, that the scheme will reduce the supply of social housing, push up the housing benefit bill, and land the UK with another property boom. That, amid all the toe-curling stuff about the new workers' party, is the heart of the matter.

Everything else attempted by Mr Osborne and his Prime Minister has failed. Austerity has been a dismal flop even by the standards set by its promoters. Now, without a hint of apology, they behave as though happy days are here again.

But here's reality: Mr Osborne still wants the £30bn in spending cuts he proposed in January. Labour, lest we forget, voted for that absurd programme.