The 71-year-old, who has been associated with the English Premier League club for more than half a century, said he was confident United manager Alex Ferguson would find suitable replacements for the Portuguese forward.
"It's a lot of money, it's crazy really," the United director told a news conference on Thursday to announce the club's partnership with Chinese electronics firm Aigo. "If you want to be in the race, you have to pay the price ... it seems sometimes a little bit vulgar.
"But nevertheless that is business, I'm afraid. We have responsibilities to our company and to all the fans that support Manchester United and it was decided that Ronaldo could go ...
"But it's a little bit soul-searching, you never know if you've made the right decision until the start of the season.
"It's an enormous amount. We will make sure we will use it properly and our club will be better for it."
Charlton, who scored 249 goals in 758 games for United from 1956 to 1973, said he thought Ronaldo would eventually rue leaving the three-times European champions for Spain.
"He is a great player," said Charlton. "He's been with us for five or six years and in that time he has improved beyond all recognition.
"I suppose, in time, he will reflect, and say, that the best time he ever had in football was when he was at Manchester United. It was better for him and it was fantastic for the club.
"But we have no divine rights, we talked him out of it for one year but he's gone and we move on."
Ronaldo has scored 118 goals in 291 appearances since he joined United in 2003 but Charlton does not think he is irreplaceable.
"Ronaldo leaves us because as a young boy he always dreamt about playing for Real Madrid, so if he leaves us, it's with our blessing," said Charlton, a World Cup winner with England in 1966.
"Instead of him? I'll have to leave that to Alex Ferguson. He is a great judge of a player, he's very brave with his decisions, he's very decisive with his decisions and that's why he's been a great success.
"Who the players will be I don't know, but they will be worthy of putting on the shirt for Manchester United."
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