So while watching the preview for the new Spartacus season, I did a google search of Marcus Crassus' net worth at the time. And with this came a list of the richest people of all time. With Crassus leading the pack by far with a 2 trillion dollar net worth. Keep in mind this is translated to todays currency.
http://www.instash.com/top-10-richest-men-of-all-time