You might be interested a in past post on this topic:

• How rich was Crassus?
  ^(10 Sep 2014 | 38 comments)
  ^(/u/XenophonTheAthenian describes Crassus's wealth compared to his contemporaries, with more elaboration further down in the comment thread.)

You can ask followup questions by posting your questions in the comments with experts tagged to direct questions to them.

I would say he wasn't, basically he had 200 million sesterces and an income of around 12 million sesterces a year. The best way to compare fortunes across time is not tons of grain (since machinery means grain is relatively cheaper to produce now for example) or just adjusting for thousands of years of inflation; the best way is to simply see how many wages you could pay with your money. In that case we can calculate that Crassus' income was enough to hire 32000 romans for a year. To put it in perspective, to hire the same amount of people in the US today would cost around 1 billion dollars.

Bill Gates' fortune generates 2.5 billion dollars a year in interest alone.

Source (I don't know how to highlight a specific paragraph on google books)

Another interesting point, the richest man in history using this definition was not Bill Gates but Mexican millionaire Carlos Slim since wages in Mexico are lower.