Lucky Coin

This accessory is dropped from Pirates in a Pirate Invasion.

Extra coins have a (20%) chance of dropping every time you hit an enemy with melee, ranged, magic or summon attacks. Damage return effects like with Turtle armor and Thorns Potion won't drop extra coins, nor will hitting background animals like bunnies.

From the 20% chance, there's a 89.1% chance it will be copper coins, 9.9% - silver coins, 1% - gold coins. Every time it drops between 1 and 10 coins. Thus, on average, 6.09 silver coins will drop for each time the effect happens, or 1.2 silver coins per hit.

A good way to take advantage of this is with the Slime Staff. Against stronger enemies (like Mimics), the baby slime summoned by the staff will attack rapidly, but only do 1 damage per hit. This gives more chances for coins to drop.

Another choice, whilst hunting for money, is to use the Flask of Gold or Golden Bullets in addition to the Lucky Coin, increasing the amount of money enemies drop after they die.

Trivia

 * The coin may be a reference to Scrooge McDuck, as when he used his coin, he was always able to make massive profits.