In Gears of War 3 for the Xbox 360, you can unlock the Pinata mutator, which will make enemies drop coins when they are killed, and has no other negative or positive effects.
To unlock the Pinata mutator, you need to earn the Gold Investor medal by earning a total of $500,000 in Beast mode.
Check out my other guide if you’re looking for the full list of unlockable mutators in Gears of War 3.