The Portuguese forward spent six years at Old Trafford before moving on in 2009 to Real, where he has continued to enjoy success both individually and as part of the team that last year wrested the Primera Division title away from Barcelona.
The 27-year-old became the most expensive footballer in history when he moved to the Bernabeu for £80million and Ferguson, while acknowledging he would welcome Ronaldo back with open arms, knows he is beyond United's reach.
Asked if the door was always open for the player, Ferguson said: "I'd like to think it would be but I don't think it will happen.
"You're talking about incredible amounts of money now. What would you think his value would be? He's definitely unbuyable.
"Who could afford to buy him? There's talk of the Russians, perhaps. But do you think he would go to Russia?
"He complained about the weather here, let alone the middle of winter in Russia.
"He was definitely good value for Madrid but he always wanted to play for them, it was his boyhood dream."
Ferguson regards Ronaldo as the best player in the world alongside Lionel Messi and thinks he would pip Barca's Argentine superstar to more individual accolades if Real can win the UEFA Champions League.
"He's an unbelievable player. One of the best. No doubt about that. I know we have had a great history of fantastic players but there is no question Ronaldo compares with any of them," said Ferguson.
"Barcelona have had more success than Real Madrid. They have won Champions Leagues and I think that matters. If Ronaldo won the Champions League he would be the player of the year.
"Yes, if Real won the Champions League he would be the main player. He was only 24 when he left us and his goalscoring record is quite amazing, even for us. In his last season for us he made five chances every match. Five opportunities created all by himself."