Would you say it is: a) a gigantic foundation stone for our future prosperity that will boost growth, create jobs, raise living standards and increase productivity; b) a terrible deal that advantages big American multinationals at our expense, or c) not a big deal either way?

Malcolm Turnbull and his ministers' exaggerated claims about the benefits likely to flow from the agreement rest on the expectation that it will allow our farmers to sell more sugar, beef, cheese, wool and rice to the other economies, at higher prices.

Since these gains have been achieved at little cost in terms of increased access to our market for the other countries' exports, we're surely well ahead on the deal.

Does that make sense? Only if you don't know much economics.

Downside exaggerated too

As for the claim that it's a terrible deal, it has some truth to it, but is itself exaggerated. It's true that part of the deal involves our acceptance of "investor-state dispute settlement" arrangements, which allow foreign companies – but not local businesses – to take actions for damages against governments that make decisions which adversely affect their profits.

This is an unwarranted imposition on democratic governments' sovereignty which, at best, will involve them in significant legal costs in fending off vexatious claims.

It's true, too, that trade deals with the US have involved attempts to advantage American companies holding intellectual property – patents, copyright and trademarks – at the expense of local consumers.

And the intense secrecy in which the TPPA has been negotiated – we still don't know the details of the agreement and won't for some months – raises justified suspicion in many people's minds. What is it that big companies and lobby groups may know, but the public may not?

Even so, it does seem that Trade Minister Andrew Robb has fended off American attempts to further advantage foreign pharmaceutical companies at the expense of Australian patients and taxpayers.

Deals restrict governments

In the old days trade agreements were about increasing trade between countries. These days, they're at least as much about imposing restrictions on governments' freedom to legislate as they see fit.

But to assess the likely effects on the economy – on growth, incomes, jobs and productivity – we need to set this legislative aspect to one side and focus more directly on trade and investment.

Be clear on this: there's no doubt that reducing barriers to trade between countries increases the material prosperity of the countries involved. Reduced protection and increased trade played a significant part in the greater prosperity enjoyed first by the developed economies and then the "emerging" economies since World War II.

Most of those gains were achieved by successive rounds of multilateral reductions in import tariffs and quotas. That is, the reductions applied to all of a country's trading partners, not just some.

But the World Trade Organisation has been trying unsuccessfully since 2000 to organise another multilateral agreement. In the meantime, countries have taken to making bilateral trade deals, where the concessions made to the other country aren't available to any other economy.

This makes them preferential trade agreements, not the free trade agreements they are known as.

Inferior deals being cut

They're greatly inferior to multilateral agreements because they tend to divert trade from more efficient to less efficient supplier countries, simply because the less efficient suppliers happen to be subject to lower import duties.

And the picking and choosing between which countries get preferential treatment and which don't creates a need for complex "country of origin" rules that add much red tape to international trade.

This week's regional preferential trade agreement between 12 countries representing 40 per cent of world gross domestic product will still be trade-diverting to some degree, particularly since it excludes such significant trading partners as China, India and Indonesia.

But it could lessen the burden of red tape if, as mooted, it involves uniform country-of-origin rules.

The other weakness of trade deals is their encouragement of mercantilist thinking – the notion that countries get rich by exporting as much as they can and importing as little as they can – a fallacy economists have been fighting since the days of Adam Smith.

The nature of bargaining is to gain as many concessions as you can while making as few of your own as you can. But this is the exact opposite of the way you maximise the economic gains from trade.

No big deal

You gain most not by inducing trading partners to reduce their barriers to your exports, but by reducing your own barriers to their exports. You gain when you shift productive resources from things you aren't very good at doing to things you are.

That's the first reason for believing a modest increase in sales for our farmers and little change for our import-competing industries won't do much to increase growth, jobs and productivity.

The second reason – and another reason mercantilism is fallacious – is that if we did get a lot higher prices for our agricultural exports without much change in import prices, this improvement in our terms of trade could be expected to lead to a rise in the value of our dollar.

If so, our farmers might be better off but this would be at the expense of our manufacturers, tourist industry and other exporters of services.

As yet we've done no modelling of the likely economic benefits of the TPPA. But various American modelling exercises – and our officially commissioned modelling of our recent bilateral deals with South Korea, Japan and China – all suggest the gains will be small – say, a level of GDP that's just 0.5 per cent higher than otherwise after 10 years. No big deal.

Twitter: @1RossGittins