This achievement is a lot less daunting than it may seem. I suggest playing normally until you decide you want to grind this to save time in the long run. As this is cumulative, you can just buy something from a store and sell it back for the same amount of money repeatedly to make your progress go up. JUST MAKE SURE NOT TO LEAVE THE STORE. I did this accidentally and the item I just bought for 60000 went down to 4000.