President Trump embarks on a visit to Saudi Arabia on Tuesday with a $1 trillion wish list for investments in the United States — a sum equivalent to the kingdom’s entire gross domestic product last year.

The Saudi crown prince is offering $600 billion during Mr. Trump’s presidency.

Neither figure is realistic, economists say.

With a laundry list of its own costly megaprojects to build and with oil prices well below the level needed to finance its spending, Saudi Arabia is facing a budget deficit that could balloon to more than $70 billion this year. It is increasingly borrowing money rather than lending it.

But that did not stop Crown Prince Mohammed bin Salman, the de facto leader of the authoritarian Saudi monarchy, from pledging in January to invest $600 billion in the United States while Mr. Trump is in office. Not long after that, Mr. Trump called him a “fantastic guy.”

Then the president said he would ask the crown prince to round that figure up to $1 trillion. And bring down the price of oil — the source of the kingdom’s vast wealth — to boot.

“We’ve been very good to them,” Mr. Trump said.

Economists said that Saudi investments in the United States would probably increase during Mr. Trump’s administration, but were unlikely to even approach $1 trillion.