It is no great surprise that the Government could be £1 billion adrift on its forecasts of what the Olympics sites could fetch after the London Games have been held in 2012. To try to guess property prices ten years ahead is an almost impossible game. Nevertheless, the assumptions made by Tessa Jowell and Ken Livingstone about site values were made at the peak of the property boom, and now look wilfully optimistic.

Errors of optimism are in the nature of Olympic projects. There are three variables in any construction project: time, quality and price. The Olympics cannot be delayed beyond 2012. The world's athletes cannot run a 100 metres on a 90- metre track. The only thing that can move is the price. That makes cost control doubly important.

This is not only a matter of pounds and pence. It would be a tragedy if the nation's flame of passion for this heroic endeavour were to flicker and fade even before the Olympic torch begins its stately progress from Beijing. The surge of elation felt around the country when London beat Paris reflected the hugely exciting prospect of our capital playing an historic role. But if budgets are constantly being revised upwards, if new information keeps coming to light, there is a risk that some citizens may become embittered, rather than elated, about an event for which many fear they will have to foot the bill. The public need to feel confident that the authorities will spend money efficiently, and deliver the best possible Games at the lowest possible cost.

To that end, citizens deserve a clearer explanation of the balance sheet. The Olympic Delivery Authority (ODA) estimate of the bill for building the facilites now stands at £9.3 billion. About half of that money is being provided by the Exchequer, about a quarter by the National Lottery and the rest through a combination of the taxpayer-funded London Development Agency (LDA) and higher London council taxes. Contrary to rumour, the average London council tax bill is set to rise by only £20 a year. If site sales after the Games deliver less revenue than expected, as seems likely, the chief losers will be sports and arts organisations, which could otherwise have gained funding from the lottery or LDA. The arts community is already protesting vigorously.

The budget for staging the Games is £2 billion. About a third of that will be injected by the International Olympic Committee (IOC), out of broadcasting rights and sponsorship. The rest must be raised from private sponsorship through the London Organising Commitee of the Olympic Games (Locog), chaired by Lord Coe. Revenues from ticket sales and selling merchandise will accrue to Locog, not to taxpayers.

There needs to be more transparency about the assumptions that are being made which underpin the numbers. It is clear, for example, that the Government is the guarantor of last resort in the (unlikely) event that Locog should fail to break even. It is better to be open about these issues. But the division of responsibility between Ms Jowell, Lord Coe and the ODA obscures them. The public would be in a better position to judge the efficiency with which these bodies are managed if there were one individual who was clearly accountable.

The Olympics dealt a severe blow to Greece's reputation because of mismanagement, while they left the world in love with Australia. The London Games should be a spectacular success. Make transparency the watchword, and that outcome becomes even more likely.