50 Cent: Blood On The Sand

The money that goes towards this achievement does not accumulate over the course of the game. In other words, you must reach $1,000,000 cash without spending any of it.

The best way to get rich quick is to go through Mission 8, Wilder's Crib, and choose the last checkpoint "Showdown". Make sure that you restart from the last checkpoint once you find the first seven boxes that are easily accessible in this level.

Locations of the money:
3 boxes in the room in which two soldiers burst from at the very beginning
2 boxes below you, turn around as you drop down straight infront of the room with 3 boxes.
Drop down to the right and collect the single box hiding in the alcove
Make your way to the other side of the stairs and collect the single box hiding in the alcove

Credit to Xbox360Achievements for the money location information.

You need to have the $1 mil in the bank, so you can go through the entire game without buying any weapons/taunts/counterkills while finding and smashing every money crate and rushing every enemy to get money drops - if you want to play that way.

Faster/earlier way to get it. Mission 4, Chapter 2. Once you drop down to get the second C4 point, just use the rocket launcher to explode the money crates in the cement tube, collect them, and then load checkpoint. You get about 10K per load, and it takes less than a minute. Repeat 100 times. Load the game to the HD to reduce load times.

Credit to Cheezbrgr for hint and video on x360a.

Bearing in mind that this achievement isn't accumulative and that you need to have a million dollars in the bank, you'll need to play through nearly the entire campaign without purchasing any upgrades or unlockables, however there is an incredibly easy method to farm money on this game!

Once you reach Mission 4: Chapter 3, proceed to where you need to plant the second lot of C4 and then smash the crates in the cement tube (as seen in the video below) and collect the money, then reload from checkpoint and do it again, rinse and repeat!

You get about $10,000 per reload so you're looking at a whole bunch of reload from checkpoints, but this is the quickest way of getting this achievement and you should have it within 30-40 minutes, to help reduce loading times install the game to you HDD if you have the space!

While you're working toward this achievement, you might as well grind this out a little extra so you have enough cash to buy all the in game upgrades as they have achievements too, you need somewhere in the region of $1.5 million for all the upgrades!

I've linked in a video showing you this method so you can see it for yourself and make sure you're doing it right!

* ^{Credit to xCheexbrgr for the video}

Mission 7, Chapter 4. Right at the very beginning, go to the left and there are 2 boxes containing a total of $17,000. Smash, reload and repeat. Takes 20 seconds per go.