The Billfold takes a shot at answering the age-old question.
Calculating his velocity (roughly 5 m/s2) suggests that this mountain (of which we cannot see the summit) has a slope of 35 degrees, putting a rough estimate of the entire hill at 73.5 billion. Is it possible that McDuck pushed together his wealth to make this monstrosity? In theory, yes, but the eye line of McDuck (fixed at 8 degrees above the horizon relative to the slope) suggests that there are at least two other such mounds, putting his total wealth at over 210 billion, and well beyond the meager 70 billion of richest man in the world Carlos Slim Helu. It’s also heartening to see cash in this picture, as a diverse portfolio is always a successful one.