I'm particularly interested in these two writers because Carl Barks was the original, while Don Rosa innovated heavily while staying very true to the original.
Scrooge frequently makes remarks on his wealth: it is Five multiplujillion, nine impossibidillion, seven fantasticatrillion dollars and sixteen cents according to The Life and Times of Scrooge McDuck. Unfortunately, we have no idea what that means in real life.
There have been several attempts at estimating his wealth. Forbes claims that McDuck compares his wealth to 21.4% the value of Fort Knox, arriving at an estimate of $44.1 billion. Unfortunately I'm unable to find the source for this quote.
On the other hand, this article estimates the volume of silver in the money bin and arrives at an estimate of $27 Trillion. However, it is limited by the way in which it doesn't account for Scrooge's treasures, businesses and external investments.
EDIT: Many thanks to user14111 for pointing out errors in calculation in the second link