The England No.1 joined local rivals Manchester City in 2006 from League Two side Shrewsbury Town, after United passed up the opportunity to bring him to Old Trafford.
The 24-year-old has gone on to establish himself as an integral part of the Citizens' first-team after Roberto Mancini promoted him to first-choice stopper ahead of Shay Given at the start of 2010-11.
Hart is now widely regarded as one of the top goalkeepers in the world, and is certain to be on the plane heading to Poland and Ukraine with England in this summer's Euro 2012.
"I could have bought Joe Hart for £100,000 so we all make mistakes," admitted Ferguson in a recent interview. "If you look at the England goalkeeper situation for the last 20 years, I would think he's easily the best."
Following the retirement of Edwin Van der Sar last summer, United moved for Atletico Madrid's David De Gea, who arrived at the reigning Premier League champions for a fee in the region of £18 million.
The Spanish stopper has struggled with the physical demands of the English top-flight though, and has been in and out of the first-team with Anders Lindegaard providing competition for the No.1 spot.
Ferguson is confident that De Gea will be a long-term success at Manchester United, and is hoping he will establish himself as a consistent performer for the Red Devils before he retires in two or three years time.
"I think you always want to go out on a winning note," he added. "Hopefully we can do that.
"I don't know how long I can last now but if my health stays up I don't think another two or three years would harm me."
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