US President Donald Trump has vowed "severe punishment" if agents of Saudi Arabia's rulers killed Washington Post columnist Jamal Khashoggi.

But what would that punishment look like? Would it involve sanctions, as suggested by members of Congress?

Speaking on America's 60 Minutes, Mr Trump made it clear, whatever happens, he has no intention of endangering an arms deal with the Saudis worth over $US100 billion.

"I don't want to hurt jobs, I don't want to lose an order like that, there are other ways of punishing them."

But then, he said something a little unexpected.

"There is a lot at stake, maybe especially so because this man was a reporter. There's something — you may be surprised to hear me say that — there is something really terrible and disgusting if that were the case, so we're going to have to see."

Trump changes his tune

Is this not the same Donald Trump that has repeatedly called journalists the "enemy of the people"?

And let's not forget the time in late 2015 when Mr Trump answered concerns that dozens of journalists have been murdered in Vladimir Putin's Russia with:

"He's running his country, and at least he's a leader, unlike what we have in this country. I think our country does plenty of killing also."

But it is also important to remember this is the same Donald Trump who as a candidate griped about the amount of money the US spends extending its defence umbrella to the Saudis:

"...without us, Saudi Arabia wouldn't exist for very long. It would be, you know, a catastrophic failure without our protection".

The 'Trump doctrine'

Perhaps the US-Saudi relationship is ripe for a Trumpian makeover. The US is becoming increasingly energy-independent, thanks to fracking, shale oil and renewables. Saudi oil imports are now at their lowest level in 30 years, keeping a lid on international crude prices, constricting the Saudi economy.

To an extent, what is emerging as the "Trump Doctrine" for this most transactional of presidents is pretty simple; try and get a better deal for America out of its alliances, trade deals and even its traditional animosities.

That means spending less defending, or indeed attacking others, and aggressively pursuing a better deal, creating markets for American manufacturers, including arms-makers.

But as Mr Trump's comments to 60 Minutes reveal, he is acutely aware he must not overplay his hand.

Saudi Arabia is the biggest buyer of American-made military hardware, accounting for nearly 10 per cent of US exports between 2010-2015. And the agreements signed in Riyadh on Mr Trump's first overseas trip in May last year hold the potential for $US350-billion in sales over the next decade.

That's a lot of American jobs, just the kind of deal Mr Trump was elected to deliver.

Sorry, this video has expired Turkish media publishes vision of alleged Saudi "assassination squad"

Saudis could cosy up to Russia

Saudi-owned media outlet Al-Arabiya has reported the Kingdom's rulers are weighing responses to potential US sanctions, including allowing Russia to build a military base on its soil, or even scrapping the US dollar in favour of China's yuan when pricing oil.

A Russian base in Saudi Arabia would be a major blow to American prestige in the region. Already, Moscow has lured Turkey and Egypt a few more steps away from Washington and shown the limits of US power in Syria through its efforts with Iran to prop up Bashar al-Assad.

Do the Saudis really see Russia as a buffer against Iran? Does an economically weak Russia really want to take America's place in the Middle East?

Does Donald Trump want to risk losing the sale of arms, given he's the one that's been trying to drum up demand by questioning America's 70-plus year role as the global cop?

Jamal Khashoggi hasn't been seen since he entered the Saudi Consulate in Istanbul on October 2. ( AP: Emrah Gurel )

Saudi heir — reformer or autocrat?

These will be decisive questions, although the key is probably Saudi Crown Prince Mohammad Bin Salman, in whom so many American hopes for the future of the region rest. The 33-year-old apparent heir to the Kingdom is defence minister and effectively a regent, given his 81-year-old father, King Salman, has Alzheimer's disease.

The Prince has presented himself as a reformer, famously allowing Saudi women to drive for the first time earlier this year (although that may have more to do with the cost of keeping half the population from full economic participation).

Saudi Crown Prince Mohammed bin Salman has been trying to cultivate an image as a reformer. ( AP: Presidency Press Service/Pool Photo )

A sign of what's to come

Jamal Khashoggi was one of very few critics to publicly question the extent to which Saudi Arabia's rulers, including the Crown Prince, would ever allow the nation to become free or democratic. That Khashoggi's very temperate concerns were met with an apparently lethal response suggests he was entirely correct.

Jamal Khashoggi is a critic of the Kingdom's current government and its ruling prince. ( AP: Hasan Jamali, file )

Perhaps most crucially of all, the arms deal Mr Trump is so focused on is less a deal than a promise of things to come.

Importantly, the signing ceremony in Riyadh last year was not for binding contracts, but letters of interest and intent, including for the kind of THAAD missile defence system the US has deployed in South Korea and would like to deploy instead of American forces in many parts of the world.

What price does Mr Trump place on a foreign journalist potentially killed in a foreign land by America's biggest arms customer?

Maybe the cost of those THAAD missiles just went up.

We'll soon see.