How much will the Olympic Games cost the taxpayer?

The overall public sector budget is £9.3bn, but the government now says the real spend will come in at about £8.5bn, largely thanks to savings in the construction process.

That’s still almost exactly double the 2005 estimate of £4.2bn.

Oxford University researchers worked out last month that this is the biggest overrun since Atlanta in 1996, although the average trend over 50 years is for Games organisers to overspend by 179 per cent.

Is this the most expensive Games in history?

We’ll probably never know. It’s certainly up there with the big ones – Tokyo 1964, Barcelona 1992 and Athens 2004 – but we have the problem of adjusting for inflation and we have to be sure that all the costs have been tallied in the same way.

Beijing is the big unknown, and although it’s unlikely London will cost more than the 2008 Games, the Chinese government has never released credible accounts. Academics have variously estimated the cost of staging China’s first Olympics at between £3bn and £22bn.

Most host countries, including the current one, have faced accusations of not putting all the real public costs on the final balance sheet, making meaningful comparisons impossible.

What is the government not telling us?

Various critics, including the House of Commons Public Accounts Committee and, err…Sky Sports, have put the real cost to the taxpayer of London 2012 much higher than £9.3bn.

The select committee said the real cost would top £11bn, because ministers had conveniently left out the £766m it cost to buy the Olympic Park land and £826m that will be spent on various Games-related projects by Government departments.

The government replied by saying that the Olympic park money will be recouped when the park is sold off after the Games.

Sky Sports suggested the real cost is £12bn – with another £12bn of “associated costs”. The broadcaster calculated this by including various “hidden” extras that have cropped up that should be included in a final estimate of public costs.

Some of these are debatable: is the cost of paying Tube workers not to strike during the Games really a direct cost of the Olympics? Might we have had to pay them more anyway?

Others are downright baffling. No one in government is able to shed any light on the widely-quoted mention of “£4.4bn budgets of the security and intelligence services”.

Freedom of information requests suggested councils have spent more than £11m on the torch relay and other Olympic events. Then there’s the cost to local police forces of securing the torch route, not to mention the undisclosed bill for the Met Police for the 70-strong torch security team’s wages…

But will the economic benefits outweigh the costs?

It’s equally difficult to be confident about this, unless you’re the Prime Minister, who last month said: “I am confident that we can derive over £13bn benefit to the UK economy over the next four years as a result of hosting the Games.”

UK Trade & Investment have given us a breakdown of that figure. Most of it comes from overseas investment, which they hope to attract to Britain through a series of conferences and summits for business leaders. The government is also hoping that millions of extra foreign visitors will generate £2.3bn by 2015.

There’s not much to be said about any of these numbers other than that we hope they turn out to be true.

But this whole approach raises the question of “opportunity cost” or: “What would have happened if we had spent the money on something else?”

It’s perfectly possible for the government to have launched a massive investment drive anyway, even if the Olympics were not taking place, so are these benefits really “Olympic benefits”?

This is a problem that remains unanswered in many of the cost-benefit analyses that have emerged in the run-up to the Games.

Mr Cameron quoted a study by Lloyds Bank which predicts a staggering £16.5bn contribution to GDP from 2005 to 2017. Most of that comes from the construction projects and the supposed boost to tourism.

It must be right that building the Olympic park will create jobs and the money from workers’ wages will trickle down into the wider economy, and so on, but that ignores opportunity cost.

But as academic sceptics point out, you might have created a bigger stimulus to the economy by building roads instead, or by just handing out the money to people for them spend.

Tourism is a tricky one, too. While extra visitors will undoubtedly spend more cash, we don’t know how many people who might otherwise have come to London this summer might stay away because of the Games.

That’s one reason why other analysts have been more cautious than Lloyds in their assessment of the economic legacy. Goldman Sachs see an increase in GDP of 0.3 to 04 percentage points in the third quarter of this year but thinks will be “largely reversed” in the next quarter. IHS predicts an almost identical pattern.

Goldman does think there will be long-term benefits but says it’s impossible to put a price on them.

How many medals will we win?

This one’s a bit easier. The answer’s 62. Fact.

Well, not exactly, but it’s our best guess, based on an overview of all the prediction models that have been crunching the numbers for the last few weeks.

We looked at forecasts by the University of Salford, Goldman Sachs, PWC, the Tuck School of Business, Sports Illustrated, the Wall Street Journal, US economics professor Dan Johnson, Ruhr University Bochum, USA Today and Sports Myriad.

The economists ignore sporting form and look at things like population size, GDP per capita, climate and the advantage of being an ex-Soviet state, many of whom punch above their weight thanks to historic investment in sport.

Most importantly for Team GB, there is also a home advantage to be factored in, which can see a host nation bag 54 per cent more medals than it would otherwise have hoped to get, according to one study.

Other analysts used past performance as an indicator. We looked at ten different predictions and took the average, which works out at 61.5 medals overall and 24 golds.

That would be the best Olympic performance for Britain in the modern era. You would have to go back to 1908, the first London Games, when only 22 nations competed, to find a better score.

So it’s over to you, Team GB. No pressure.

By Patrick Worrall