Life is hard in Columbia and saving up enough Silver Eagles to make life a little easier is just as hard. Luckily, if you happen to be in a pinch and really need that money, there are some really easy exploits to get your hands on some. Playing the story missions get you money faster than taking advantage of the exploits, but you might not always be able to take that route for whatever reason.
There are many re-spawn locations you can take advantage of but a good one is in the Hall of Heroes. There is a vending machine to the right side of the stairs leading to the entrance of the Hall of Heroes. Possess the machine to get some quick cash.
Enter the Hall of Heroes and go down the hallway to refill your salts. Return to the vending machine to get some quick money again. You can repeat this indefinitely.
Furthermore, enemies, containers and loots also re-spawn so you can get even more money on each trip.