Rockefeller's wealth peaked at between $1 and $2 billion, according to the best records on him, including his biographies and records from the Rockefeller Archives. Inflation adjusting those numbers based on the 1890-1910 time frame in which the dollar was extremely stable for the most part, you get between $20 (20x$1b) and $60 (30x$2b) billion, depending on if you want to use the low ball on his wealth plus a low ball on the inflation adjustment vs the opposite end of the spectrum.