This can be earned throughout the course of 3+ playthroughs of the game.
A complete playthrough is worth $395,000 if you collect all briefcases and complete all bonus objectives. Assuming you have already collected all briefcases in the 1st playthrough, the 2nd time you can only earn $288,500 max. 3 playthroughs will therefore net you $972,000. You can spend money you earn as the game tracks your total earned, and not what you currently have.
Note: Objective rewards are the same in any difficulty.
Most of the sub-objectives are pretty straightforward except for one in China where you are asked to take down the first Chinook. Preparation is needed beforehand:
You need either the RPG or Stinger (FIM92). I recommend the Stinger because despite the damage chart, it does more damage than the RPG (2 shots to take down the Chinook instead of 3 RPGs), it comes with 3 ammo by default and it has lock on. The only con is that it cost nearly 5x more.
Once you dock at the dam, enter mid-mission shopping if you haven't already and equip/buy the Stinger ($42,500). Throw the first control switch and clear out all the enemies first before moving the hovercraft in. After climbing up the ladder, clear out any remaining enemies again. Once you throw the 2nd control switch, the Chinook comes flying in across the dam. Lock on and take it down with 2 missiles before it flies off.
Go to Mission Select once you've completed the game and choose level China and complete the first objective (Blow up the bridge the convoy is passing over), run back to the hovercraft at the bottom to trigger the checkpoint (else the money won't be credited) and then quit the game. It takes less than 2 minutes and it's worth $30,000. You can rinse and repeat till you get 1 million.