That was mentioned in the book "The Millionaire Next Door" by Thomas Stanley. Many wealthy people dress and live modestly and go about quite unnoticed by people.
Recent stats show that half of all Americans fail to save adequately for retirement, but I've yet to see a stat on the numbers of Americans (mostly above 50) who have net wealth of at least half a million. I'll bet it's very substantial.
The late Mr. Stanley’s classic ‘Millionaire Next Door’ should be required reading in high school if not before.