The Colombian claimed he could have earned more by staying at Williams as he played down talk of a £15 million salary when he makes the switch in 2005 at the expected expense of Britain's David Coulthard.
The 28-year-old said the motivation was to further his grand prix education but is determined to bow out of his current contract in style by achieving his ultimate goal this season of winning the world championship.
"People say I will be getting £20m but everybody would be surprised at what I will be earning at McLaren," said Montoya in Valencia yesterday as he launched the FW26 car.
"I could have earned more money if I had stayed here and we had talked. But this has nothing to do with the money. I have got enough money to live with anyway.
"It was an emotional decision to leave because I am very close to Frank Williams. I have a very good relationship with him but at least know I know where I will be driving for the next five years."
Montoya did admit, however, that he was annoyed at the disparity in his salary compared to team-mate Ralf Schumacher's reputedly around £6m especially as he has outscored the German for the past two seasons.
"I have never been happy that he has had more money than me and I was beating him," added Montoya, who finished third in last year's drivers' standings, 24 points ahead of fifth-placed Schumacher. "But that's not the reason I'm leaving. When Ralf did his deal I was struggling a bit with the team and the car and he took that opportunity to get a lot of money out of Frank. It is just business really."
Montoya had been linked with a switch for this season, but the Bogota-born driver who joined Williams in 2001 having won the IndyCar title in the US two years earlier never had any intention of moving early. "I did not want to go for 2004 because I think Williams have a better chance of winning the championship this year than McLaren," added Montoya, whose manager was first contacted by McLaren boss Ron Dennis around six months ago.
Montoya has also rubbished claims by Ferrari's sporting director Ross Brawn that the early announcement of the move will destabilise his relationship with the team who will not want him testing sensitive 2005 parts next season. "If I was sitting in Ross Brawn's shoes and I was the biggest threat to Michael Schumacher last year and this year I could be a bigger threat, then I would be saying things to wind everybody up," he added.
"While I am a Williams driver I will give it 100%. We're here to win, simple and that's the aim. The team will be committed to me, anybody who knows Williams knows that there main priority is winning the constructors' championship."
Montoya is hoping the striking nose job on the FW26 which features a radical catamaran-style twin-keeled front end will prove not to be a stinker as he bids to end Schumacher and Ferrari's F1 dominance.
"If the car works as good as it looks then we are in good shape," he added.