The Portuguese forward left United to sign for Real Madrid in 2009 for a fee of around £80m.
United have been linked with a bid to bring Ronaldo back to Old Trafford in recent months, but Ferguson believes that the forward is worth far more than the £80m Madrid signed him for and has ruled out any chance of a summer bid.
He told The Sun: "If you were to buy him back, you are talking hundreds of millions.
"That rules everyone out, including Barcelona. I just can't see it happening. Everyone talks about that but I don't see it. Cristiano has more than two years on his contract and I think Madrid are trying hard to get him a new contract.
"He was still young when he left us. We should have asked for £150m. We would have got it, probably. It was obvious he was going to be the top player in the world."
Ronaldo comes up against his former club for the first time on Wednesday when United take on Real Madrid in the Champions League.