The Portugal international joined Real Madrid from the Premier League giants in the summer of 2009 for a world-record transfer fee of £80m.
"I'd like to think it would be, but I don't think it will happen," said Ferguson when asked if United would be open to the idea of having the 27-year-old back.
"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!"
Ferguson, though, believes that Ronaldo will leave Madrid at some point in his career.
"He was definitely good value for Madrid, but he always wanted to play for them," said the 70-year-old.
"It was his boyhood dream. I'm not saying he'll stay there forever, you never know. How do you know we've not got a buy-back clause for him?"
Ronaldo won La Liga with Madrid last season.