Ronaldo went to Real three years ago for a world record £80 million.
"There is still value in the market if you are going to be that successful," Ferguson said. "Ronaldo has scored more goals than he has played games. At the time we thought £80m wasn't bad business, but if Real Madrid had known how good he was going to be they would probably have paid £160m.
"He's been such a fantastic buy for them, maybe we should have asked for £800m."