Ferguson has never been afraid to spend big money on key players.

The arrivals of Juan Sebastian Veron and Rio Ferdinand to Old Trafford broke previous British transfer records and only this summer he splashed out £30.75million on Dimitar Berbatov.

But even the Manchester United boss has been taken aback by City's offer.

It is not the team doing the buying that has caught him off guard, or even the fact AC Milan appear willing to sell.

For Ferguson, it is the sheer enormity of the sum, well over double the most any club has paid for a player in the history of the game - Zinedine Zidane's move from Juventus to Real Madrid for £46million in 2001 - that has left him gob-smacked.

"I find it hard to get my head round to be honest," he said.

"It is amazing.

"Football is football. From time to time you get shocks and surprises. This is surprising everyone."

Ferguson is not convinced the staggering deal would have a knock-on effect throughout the game if it goes through.

United midfielder Michael Carrick, who left Tottenham for a not inconsiderable £18.6million in 2006, has no view on whether lavishing such sums are morally right or wrong.

However, it is clearly a move that has got the United dressing room buzzing, with the England midfielder left as stunned as his manager.

"That amount of money being bandied about is a bit of a shock really," he conceded.

"When Manchester City were taken over we were told they had a lot of money to spend and big names were being talked about.

"But for it to be so much over the top is pretty mad really."

While AC Milan seem happy to do business, City owner Sheikh Mansour knows there is a lot of negotiating to do with Kaka and his advisers before the Brazilian commits his future to Eastlands.

A key meeting between the Blues and Kaka's father has been pencilled in for next week, after which the picture will become clearer.

Carrick is not certain Kaka will eventually sign. But the former West Ham star would be delighted to see the former world footballer of the year in the Premier League, even if it was in the shirt of United's local rivals.

"We want the best players in this league and he is definitely one of them," said Carrick.

"It will be interesting to see what happens."

Wigan manager Steve Bruce has pointed to the City offer for Kaka as another warning that football is in danger of losing touch with the man in the street.

With City said to be prepared to offer AC Milan striker Kaka £500,000 a week and ticket prices continuing to rise while the economy heads in the other direction, Bruce fears for the future of the national game.

Bruce, who takes his side to Eastlands tomorrow, is also worried the move could drive up player costs to unsustainable levels.

He said: "The beauty of our game is that it means more to the average man in the street than anybody. I know the average man in the street now finds it very difficult to find £40-45 to go and watch a game.

"When a big player is out there it does inflate your prices. You do worry about it and wonder if things could go belly up."

Bruce added: "We are all staggered by the news. It is quite unbelievable when you are talking about a credit crunch throughout the world. But it just shows you what this Premier League is all about.

"I think this year people thought the situation with finances would be a little bit more sensible.

"But then along come the owners of Manchester City who want to have the best players in world at their club."

However, Blackburn manager Sam Allardyce, who last week rejected a £16million bid from City for striker Roque Santa Cruz, believes the massive investment City are prepared to make could pay off if they break into the Premier League's top four.

"There is only Manchester City that can spend it (£100million on one player)," he said.

"Manchester United can't, Liverpool can't, Chelsea can't now by the looks of it.

"The last few years it was (Chelsea's billionaire owner) Roman Abramovich and now we have got another one.

"If there is going to be more to follow then only time will tell if spending that sort of money is the right thing to do.

"If Manchester City end up challenging the big four then from their point of view it will be."

Chelsea manager Luiz Felipe Scolari, who worked with Kaka during his time as coach of Brazil, offered no opinion on the size of the fee but insisted City would need more than money to convince him to sign.

"Manchester City have money. Kaka is one of the best in the world on the pitch and off it. He is a fantastic player and they have money to spend. How much is it normal to pay? I don't know. He is a fantastic player," Scolari said.

"I would not be surprised if he went to Manchester City. This is football. He is a professional player but maybe they offer to him not only money but an idea for the future. I know Kaka very well, money is not his problem."

Fulham manager Roy Hodgson, former manager of Milan's city rivals Inter, said: "You're in the realms of lottery money and that's the way it is. If a club has £100million to spend and think that buying one player is the best way to improve their team then that's their business.

"Richer clubs have always determined the market prices. At the moment £30million is the figure for a very good player to move from one Premiership club to another. In a few years that figure might be £100million."