To easily gain money, destroy an ATM machine. Each one will give you approximately $2,000.
To easily gain money, find a slot machine that has see a pile of $100 in front of it. You can win on that slot machine up to five times. Go to the map by entering any casino or the machines by the Sports Car in the Royal Flush Plaza. The machine will then give out winnings again.
To easily gain money, have at least $25,000 to $50,000. Go to the Slot Ranch casino. On one of the walls is a Heart slot machine that has two buy in amounts for $100 and $1,000. Select $1,000. Keep playing this machine until you have $6 million. It will not take very long. Then, buy as much as desired. The best combinations (hearts fill in for the third slot), three hearts are $100,000; two hearts and a dollar sign are $100,000; two hearts and a diamond are $50,000; and two hearts and a seven are $25,000.