This is unlocked upon having 1,000,000 coins at once, it is NOT cumulative. So I suggest to not spend anything for the entire playthroughs ( all the way to Ending E ), complete all side quests which nets quite a sizeable amount of Money. When you have finished Ending E, save the game and go to a shop, list everything to sell and see if you reach the required 1 mil, if not there is 2 options.
1) Complete the 3rd Recycled Vessel door ( which is unlocked after finishing the game, its a diary on a shelf in Niers house ). Finished this will give you 50,000 everytime you run it and can be easily completed within 15 minutes if you select easy as your difficulty.
2) You can start the long and tedious grind of gathering materials for upgrading all your weapons. This is more productive as you are working towards finishing another grind as you gather enough money for your achievement.
When you get your achievement you can just reload your save so that you still have all your materials.