Tottenham and Madrid have reportedly opened negotiations over a potential world-record 114 million euro move for Bale, with the winger said to be keen to move to Estadio Santiago Bernabeu from White Hart Lane.
Calderon, who was president for three years between 2006 and 2009, believes the transfer will be completed this summer, not least because current Blancos supremo Florentino Perez is concerned Ronaldo will not renew his contract.
"I know that [Ronaldo] is not happy at all with the attitude and behavior of the president," Calderon told TalkSport. "He is asking for 20 million euros net every season, which is going to be difficult for Real to accept.
"I think the president has all his eggs on the table just in case he can’t get the renewal of Cristiano. That is my impression.
"He's afraid at some moment that Cristiano will say, 'I am not going to go on.' That will be bad news for Madrid.
"I would like to see him retire at Real, but now the situation is difficult. Let's see in the next weeks or months if it can be sorted out and he can stay."
Ronaldo sparked speculation about his future at the beginning of last season after revealing he was "sad" in Madrid, while reports in Spain over the summer have claimed that he is still unhappy with the club's attitude to his contract renewal.