Sir Alex Ferguson says Anthony Martial's form so far for Manchester United is the perfect response to claims that his potential 58.7 million pound ($91 million) fee was 'ridiculous'.

The Frenchman arrived at Old Trafford from Monaco on transfer deadline day for an initial £36M ($55.9M), but United could yet be forced to pay a further £22.7M ($35.3M) depending on various add-ons.

While the total cost has been considered by many observers to be extraordinarily high for a 19-year-old, Martial has responded by scoring three goals in his first three games for the club, with a double helping United overturn an early deficit to beat Southampton 3-2 in the Premier League on Sunday.

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When asked at the launch of his new book 'Leading' whether Martial's price tag was ridiculous, the former United manager laughed off the notion.

"Ridiculous? Not at the moment it's not!"

The 73-year-old explained that United's overall worth has changed remarkably, allowing the club to speculate more than during his formative years at Old Trafford.

He said: "When I first arrived at Old Trafford the club had to rely on gate receipts -- imagine that! We signed Viv Anderson and Brian McClair the first year for £1M ($1.6M), then we signed Mark Hughes [for £1.8m]."

In 1986 the club was said to be worth around £10M ($15.5M), whereas current estimates suggest United is now worth over 2 billion pounds ($3.1 billion).

Ferguson's new book, which focuses on the art of leadership, is released on Tuesday.

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