The analysis gets more complex, the data less reliable, the assumptions more heroic and the result is a number describing something that might not happen anyway.
But treating a Cup win as an option is different. Ask "what should we pay to own the option - which is a chance, not a certainty - of increasing GDP growth by, say 1 per cent or $2.18 billion over the three years it is held?"
It costs around $130 million to mount a challenge. So the bet is $130 million down for a possible $2.18 billion win. This is risky. The volatility on earnings in the marine and its supporting industries is above 50 per cent. So what should we pay for this option?
The option pricing model says, if this is the prize, it would be rational to sink about $700 million right across the economy - that's $130 million mounting the challenge plus the related resource which is committed (and so unavailable elsewhere). The money could be public or private. At present $36 million of it is public.
The prize in this case might be an extra 1 per cent in GDP growth worth $2.18 billion. Roughly a 3 to 1 payoff. No guarantees, plenty of risk but a realistic way to think about what a rational spend is.
I have no idea what the value of winning the America's Cup is, but am convinced the options which winning creates for new jobs in greater numbers across more industries are enormously valuable. For a 1 per cent GDP increase they are worth spending $700 million.
I can't see many other government expenditure beats that offer anything like a 3 to 1 payoff for anything like this prize.
Brent Wheeler is an economist who is the principal in Dunedin-based Brent Wheeler Group.