"Ronaldo will be paid more than 200 million euros ($214 million) for the [al-Nassr] deal," said the source, with knowledge of the deal but who requested anonymity.
And, the source said, "He will be an ambassador for the Saudi World Cup 2030 bid for another 200 million [euros]."
The Saudi kingdom, along with Egypt and Greece, is bidding to host world football's 2030 showpiece World Cup tournament, and Ronaldo's ambassadorial role will bring his reported earnings in the country to over 400 million euros ($428 million).
The Portuguese superstar, 37, unveiled to thousands of Al Nassr fans in Riyadh last week, is by far the most famous player to sign for a club in Saudi or any other Gulf country.
The arrival of the former Manchester United, Real Madrid and Juventus player came just weeks after neighboring Qatar became the first Arab nation to host the World Cup.
The five-time Ballon d'Or winner, who has also won the Champions League five times and holds that tournament's goal-scoring record, is expected to make his Al Nassr debut on January 22.
The deal was backed by senior royals who support Al Nassr, including Saudi Arabia's de facto ruler Crown Prince Mohammed bin Salman -- widely known as "MBS" -- the source said.
- 'International spotlight' -
"MBS (and his brothers) Naif, Turki and Rakan, the sons of King Salman, are all honorary members of Al Nassr since before their father even become the crown prince," the source said.
"They wanted to grant their beloved club supremacy and put it in the international spotlight. The best way was to bring the best player in the world."
The deal has been financed by the conservative kingdom's sovereign wealth fund, the Public Investment Fund (PIF), the source added.
"Al Nassr and the other Saudi clubs don't have this huge fund for such a big deal. It's PIF that secured the payment," the source said.
According to another source close to Al Nassr, it was the princes who put the surprise Ronaldo deal in motion.
"It's MBS's brothers who suggested and sought to seal the deal at any price," he said.
"They are all Al Nassr fans and they wanted to prove their team really is the International," the source added, referring to Al Nassr's nickname.
Al Nassr has nine Saudi league titles but have never won the Asian Champions League, unlike four-time winners Al Hilal and Al Ittihad, who have lifted the trophy twice.
Recently the PIF has undertaken other high-profile sporting ventures, including the purchase of a majority stake in English Premier League club Newcastle United.
It has also bankrolled the lucrative LIV Golf League, which has split the elite golf world by luring away top stars from the U.S. PGA Tour and DP World Tour.