Ronaldo spent six successful years at Old Trafford, notably winning the Champions League in 2008, before leaving for Real Madrid in a world-record £80 million ($131.6 million, 93.9 million euros) transfer in 2009.
Widely regarded as one of the two best players in the world, alongside Barcelona's Lionel Messi, Ronaldo remains a popular figure at United, but Ferguson concedes it is unlikely the 27-year-old will ever return.
Ferguson was asked by a number of British newspapers if the door would always be open for Ronaldo, and he replied: "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 (in Manchester), 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."