Organized crime generates revenues in the billions annually. Vito had been in business since the 30s. Michael's adept expansion of the family's enterprises, both legal and illegal, could easily have amassed quite a fortune. Don't forget, Michael is working through a foundation and I am sure he arranged for its funding and disbursements to be done legally.
In addition to In God's Name, you might read God's Banker also.
Make no mistake, through this thread, I am learning much I never knew before. I am just sorry I didn't have some of the posters here as teachers when I was in school. If I did I would have been able to land a job where I could send millions to the Vatican.
