go to sandbox mode. Destroy all the casino ATMS and slot machines. Break into the safes and find money cases, then shoot them open. WHEN you start a new story mode game, you'll have all the money you got from sandbox mode at the start of the game.
I use a different approach and have accumulated over a million dollars in 40 minutes or so. In Sandbox mode clear the casino with the large slot machine against the wall (I forgot the name) of zombies. I started with around 75k and bet the $1000 max every time regardless of the outcome. After 40 or so minutes, I had roughly 1.1 mil. I saved my game and cruised the car for a bit, then quit without saving.
I found a nice little money trick in sandbox mode. I havent compared the time it takes to the time it takes other methods, but once you have the celebrate skill (hold the left direction button), kill enough zombies to unlock the "Crowd Pleaser" challenge in the slot ranch casino. Then start the challenge and hold the left D button for the minute. You should get the gold medal, rewarding you with 40,000 bucks. Ive done this quite a few times in a row, always getting the gold (always using the celebrate skill alone) so
40,000 bucks a minute. in an hour of this you'll amass around 2,400,000.
So if you dont wanna do the slot trick, or dont wanna have to worry about zombies, this is an easy method.
3DS FC: 0216-0893-6228 Finally finish reading my post? You're so zetta slow! Your reading skill is garbage! CRUNCH! I'll add it to the heap!
In Sandbox mode, destroy every machine in the casinos that give money. Also hunt down the atm machines and find the gray money cases and throw them at the wall or shoot them. Another good way to earn money is one machine in the casinos that I don't think can be destroyed. The cash booth game. Step inside and pay 100 dollars to play. You just keep pressing the X button(PS3) as fast as you can within a few seconds to raise the cash meter on the left of the screen. You will always win thousands and the cost is only 100 dollars to play each time. I did this in Sandbox mode because I was extremely low on time to pay the 1,000,000 ransom for Rebecca.