You will need at least $20 million available. After completing the story, open the internet browser, go to the "Money And Services" tab, and open the LCN stock exchange website. From there, invest all your money in stock, and then quicklysell them. You will not lose any money if you sell the stocks fast enough. Keep repeating this until you get the "A Lot Of Cheddar" achievement. Note: You can also do this earlier in the game with less money, but it will take longer. To see how much money you have already spent, go to "Cash" in the statistics of the pause menu.