Firstly, Trump estimates his own wealth at 10 billion dollars. Most of this wealth comes from his own estimate of the value of the "Trump" brand, not any actual, material wealth.

Secondly, 500 mil dollars in the 80s, adjusted for inflation alone, are worth 1.5 billion dollars today.

If we take buying power at a metric, instead of nominal value, and plug this into your calculation we get just over 10 billion dollars at a 5% interest rate