Mercenaries 2: World in Flames

Here's a real guide to unlocking the millionaire achievement.

Play the game as usual up until you capture the mansion. As soon as you capture the mansion go the docks located on the opposite side of the house. You will fine here a red box that once open you will receive 200k.

Continue the story as normal until you can enter the Universal Petroleum HQ after picking up the "dominator".

Once inside the gate right in front of the main door you will see next to a poll another 200k red box. head to the north side of the building and near the front you'll find a third 200k red box.

After getting these three and completing the missions on the way you should get the achievement.

For those of you who went on a rampage killing spree and got money taken away for killing civilians. Capture a special interest guy, once you drive out of the mansion after the UP mission you should be able to, any enemy you kill while trying to capture him will give you 5k plus an additional bonus on the VIP depending if left dead or alive.

Follow what Full Sillouette says above, where you collect the 3 red boxes, which will each get you $200k.

After that, continue on; I played the UP Mission where you have to rescue somebody. Once that is finished, you will have to go back to the base (mansion), and upon doing so, you will finish the contract, giving you $500k.

That will put you over the 1 Mill, and save, it wasn't until I actually saved my game (manually) that I got the achievement.

...Earn $1 million whilst connect to the EA servers is the unofficial description.

Note: You must be a Gold member on the account you're trying to get the achievement on.

So as of 13/12/2013 you might believe the money related/co-op achievements are now unobtainable, however this is untrue...

Follow these steps if like me you were glitched on the money based achievements ($1 mil/$1 bil):

1. Download the Command and Conquer 3 Demo from the marketplace.

2. Load up this demo, go to xbox live and accept the terms and conditions.
(You don't even need to be a gold member to do this bit by the way)

3. Exit the demo and load up Mercs 2, you will notice now that the game doesn't crash on the main menu anymore and you connect through the EA servers; finally.

4. Load up your $1 million save, then save over this slot and the achievement pops.

Other games to get part 2's terms and conditions:
BF: Bad Company 1 (Full Retail)

This can be quite glitchy. When I earnt $1m, the achievement didn't pop up until I completed the campaign, when a whole slew of achievements poped up, so if you don't get it right away, what until you complete the campaign.