Ever wonder how Bengals owner Mike Brown made his money? It's not that hard. You probably know more than enough to make the logical conclusion. One site published how owners made their money. What they write on Mike Brown:
Mike Brown’s father, Paul Brown, was the namesake and first coach of Ohio’s other team, the Browns. He went on to co-found and own the Bengals before leaving the team to his son. Like the Halas family, the Browns’ money comes from football. Should football one day cease to exist, the Browns would be penniless and confused, wandering through the Midwest wondering where their publicly-funded stadiums and fortune went. Luckily for them, football has continued to exist and looks relatively stable.
For the rest of the NFL, the basic guideline for being an owner is making sure that you're born lucky into a rich family, have oil, car dealerships, or other businesses during an era when team values were 500 percent less.