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This achievement is highly missable! It's strongly recommended to work on this during a lethal, powered High Chaos playthrough so that you have access to Blink/Dark Vision and do not have to worry about the risk of being caught when thoroughly searching every nook and cranny of a mission.

Right off the bat, it's worth noting that all loot sources (minus food) net coin. This includes paintings, ornamental items (bath salts, processed whale oil, tyvian ore, etc.) pick pocketing guards, silver and gold ingots, and coins themselves. It's also worth noting how many coins there are for mission:

Mission 1, A Long Day in Dunwall - 1490 coins
Mission 2, Edge of the World - 3854 coins
Mission 3, The Good Doctor - 2206 coins
Mission 4, The Clockwork Mansion - 4802 coins
Mission 5, The Royal Conservatory - 3868 coins
Mission 6, The Dust District - 3973 coins
Mission 7, A crack in the slab - 3334 coins
Mission 8, The Grand palace - 4430 coins
Mission 9, Death To The Empress - 2562 coins
This translates to there being a total of 29,799 coins in the game. For the purposes of this achievement, you must find/earn at least 18,000 coins before the end of your playthrough. During the post-mission stat board you will be shown how many coins you collected versus how many were in the mission. As a general rule of thumb, you should try to earn at least 60% of the total coin in every mission. Additionally, the Dark Vision ability, specifically its upgrade, Greater Dark Vision, will allow you to see objects through walls; Greater Dark Vision will prove to be incredibly useful when scavenging for loot sources and coin.

You can track your total amount of coin earned by entering the "Playthrough" tab under the "Stats" option in the pause menu, and then referring to the "Economy" stat. Once you have amassed at least 18,000 coin in a single playthrough, this achievement will unlock.