It's always helpful for Board members to read the novel. It can help to fill in some of the film's gaps.

From the novel:

"He [Michael] was amazed at how wealthy and powerful the Corleone family truly was. It owned tremendously valuable real estate in midtown New York, whole office buildings. It owned, through fronts, partnerships in two Wall Street brokerages house, pieces of banks on Long Island, partnerships in some garment center firms ...." Of course, that was in the latter 40s. The value of those assets would only increase tremendously through the decades. Also, Michael would have acquired additional assets

At the Senate hearings, Michael admitted to owning IBM and ITT stock. It's probable that he had more extensive equity holdings. I think we are all familiar with the IBM story of prosperity; ITT had a gigantic international presence in an array of business especially communications of all kind. So, just those two stocks would have brought the Corleones plenty of revenue. If one of the Dons admitted to receiving $50 million as a return on his casinos investment, I can imagine what the Corleones received. Add to that the illegal earnings from illegal activities (all tax free) and you can begin to understand the extent of Corleone wealth.