I should add that my figures don't property take into account the ages at which people accumulate the bulk of their net worth. For "number of graduates from top universities" I considered graduates between ages of 22 and 82, but this isn't appropriate, since most people won't acquire most of their wealth until later in life (e.g. the average age of a billionaire is 66), so the prevalence of eventual $30m+ people should be something like 2x what I said, and similarly, for members of the general population (where I included everyone, including children) the prevalence should be something like 3x what I said.

So the actual prevalences of eventual $30+ millionaires should be something like 1 in 1,000 (general population) and 1 in 100 (graduates of top universities).

As for billionaires, there are 442 US billionaires, maybe 100 million in the appropriate age range, so 1 in 250,000 of the general population becomes a billionaire, 129 billionaires at the schools that I looked at with ~250,000 in the appropriate age range, cutting by a factor of 2 to account for possible double counting (undergraduate/graduate), we get a frequency of 1 in 4000 eventually becoming billionaires. (About 0.4 billionaires per university class.)

This analysis is quite crude – one would need the actual age distribution and not just the average age.