That fee remains a world record for a player, but the Frenchman, 36 on Monday, thinks Real Madrid could possibly have to spend more than that to land Ronaldo.
And he said he would be happy to no longer be the world's most expensive footballer.
"If he goes to Madrid, it will be a big amount, but Madrid is ready to pay far more than they paid for me," said Zidane.
"This will actually lift a weight from my shoulders. Hopefully they will pay more for Ronaldo than they did for me."
Zidane, who retired from international football after the 2006 World Cup, said that he had been impressed with Russia's Andrei Arshavin during Euro 2008 so far.
"When you see their number 10 it is great," Zidane said.
"What he does at this stage is more difficult than a World Cup because it's the first big tournament he has played in.
"I was very happy to see that type of football played. He has great feet."
Zidane, who was in Vienna with manufacturers Adidas for a coaching clinic for children, confirmed that he wants to return to football but stressed that he had no imminent plans to do so.
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