This achievement is accumulative you do not need to have $1,000,000 with you. To find out how much you have earned you can press back, go to quest tab and press Y till you see challenges. It is within that list called "the rich get richer" (going off of memory).
The fastest way to get money is to always check mailboxes, cartons, anything with a green light on it. also enemies drop a lot of money. to simplify picking up items hold X to pick up all items with in range.
Now that the game has been out for more than a year there are much easier ways to get this achievement for example most new weapons are worth 9,999,999. Join anyones game and a 1,000,000+ weapon should drop soon enough, join a friend, have someone duplicate their weapons for you.