The formula for computing how gold reward is affected by Luc is very complicated. Your final gold multiplier ranges from 1x to 4x - it's pretty easy to reach 2x, hard to reach 3x and very difficult to hit 4x. These formula are rough approximations for simplicity sake (turns out 90% of the code in these computations affects the result by less than 5%).

Compute your base multiplier:

baseMultiplier = 1 + (myluk / (enemyHp / 30));

Against a few enemies:

Jousting Fox: 1000 luck per multiplier

Yellow Mother: 110,000 luck per multiplier

Red Phoenix: 400,000 luck per multiplier

That is not the final multiplier. There are a number of additional stepwise functions that increase your multiplier linearly:

+0.55 for 10,000 luck. +0.55 for 50,000 luck and +1.1 for 200,000 luck

However, at this point, your multiplier is harshly reduced while above 2;

Here is the baseMultiplier to finalMultiplier approximate mappings. The final multiplier can never exceed 4x.

1x = 1x

2x = 2x

5x = 3x

11x = 3.5x

23x = 3.75x

35x = 4x

Summary Edit

Against any enemy, 50,000 luck will get you at least 2.1x gold

Against lvl 50,000 enemies, you'd need 1.2mil Luck to get a 3x multiplier.

Against lvl 1,333 enemies, you'll only need around 10,000 luck to get a 3x multiplier, but almost 70,000 luck to reach the 4x cap.