Good question, Sonny.

It's possible that Michael was a multi-billionaire for all the reasons Olivant and Last Woltz cited. But what I find less than credible is that he'd hand over $700 million to bail out the Vatican Bank:

Great wealth always carries a certain amount of public disapprobation--the novel is prefaced with Balzac's famous aphorism: "Behind every great fortune there is a crime." In the US, the "robber barons" of the late 19th Century--Rockefeller, Carnegie, Ford, Morgan, Mellon--understood the power of money to wash away sins, and did so publicly through their foundations, charitable trusts, endowments, libraries, etc. Charity, such as the Vito and Carmella Corleone Foundation and its $100 million gift to Sicily, is good in people's minds. But, if Michael bailed out the Vatican Bank to the tune of $700 million, there'd have been a wave of suspicion and criticism, to wit: what is this gangster hoping to get out of the Vatican beyond being made a Knight of St. Sebastian? Is he trying to buy the Papacy? Is he hoping to get the Vatican Bank to invest in Immobiliare? Is he going to use his new-found "respectability" as a cover for more nefarious criminal activities? European politicians and regulators would have been forced to raise questions. It'd be exactly the kind of publicity Michael didn't need.