Rich Presidential Candidates

David Frum asks "When Did Our Presidents Get So Rich?"

Great question, and even better applied, as he does in the post, to presidential candidates:

It seems that the income gap is widening between presidential candidates and the average voter. Barack Obama in 2008 was not a person from a wealthy background, but compared to the typical voter, he was still much wealthier than say, Harry Truman was in 1948.
It would be interesting to see a chart of the relative differential between the combined incomes of the two candidates in any given year and the income of the typical voter in that year. I don't have the answer at hand, but I'd guess off the top of my head that the "narrowest years" out of the past century would be 1924, 1948, 1968, 1972, and 1976; the "widest" 1928, 1960, 1964, 2004, and 2008. But I may be wrong! Has anybody out there ever in Internet land ever considered this data point?

I'm not aware of any research into this, but one of the things it reminds me of is the story of Richard Nixon in the 1960s. Here's how Garry Wills put it.
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Thursday Niight, Nixon came to Convention Hall -- its interior seen through dazzles, veils woven by the rows on rows of lights. He was proud of his acceptance speech. It would (carefully) reveal his inner self. He came to make the only moral claim that mattered to his audience. To tell them he had made it. "You can see why I believe so deeply in the American dream." He had risen, politically, from the dead. And he had done it by the route these men respected -- by making money. Nixon had been a candidate before, and politician always; but only after his 1962 defeat did he become a wealthy man. The Checkers speech was truthful, back in 1952 -- he was poor; he was young, starting out, working hard to succeed. But then there had been failure, political defeat. And through it all he had not earned the money to be independent. Only when he became a Wall Street lawyer, with $200,000 a year from his practice, and with Bebe Rebozo to help him invest in Florida land, could he look his fellow Republicans straight in the eye at last. A campaign coordinator who worked with Nixon through the years put it this way: "Dick could not have made it to first base in 1968 without a substantial personal income. Republicans, especially those who finance the party, respect only one thing, success, and they have only one way of measuring success, money. Dick never had any money before now. He could not talk to these people as an equal, even when he was Vice-President. The thing that woul dhave kiled him with them was any suspicion that he simply needed a job. Now they knew he'd be giving up a damn good job, and good money" (282-283, emphasis his).
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I strongly suspect that Wills gets this right. Does it still apply? The recent Republican nominees who started life with little, such as Bob Dole and Ronald Reagan, certainly had made plenty of money by the time they won the nomination (Bill Clinton, on the Democratic side, too, although in that case it was his wife who was making the money, for however that works out).

Anyway, I know there's some data about Congress, but I'm not aware of what anyone has compiled about presidential candidates. Has anyone collected these data?