I believe it mostly. Roddick got a lot of cash, because he was the sole face of American tennis for years and a good looking dude, so a lot of his cash is obviously in sponsorships. Same with why agassi is ranked ahead of sampras. He was bigger in the public eye and longevity in the game helps.
I was at first the surprised about Andy Murray (As far his place in the rankings) but he is the sole great player in almost a century in a country that really cares about tennis.
One problem is now that I think about (thought about this while writing so I am leaving the above that it contradicts), the amount of money they say these people are worth is less than their prize money alone. They say novak is worth 60 mil., but he has made over 60 mil in prize money alone, and one would guess he has pulled in that much in endorsements over the years right?
Likewise, Rafa has just over 70 mil in prize money and is supposed to be worth only 80 mil.
The numbers seem fishy to me...