An update was released on Jan 19, 2012 that added a Treasure Chest to the game's main menu that, once clicked, gives the player a random amount of Monkey Money once a day (if you have a Ninja Kiwi account). You can also earn Monkey Money by completing Special Missions.
An update was released on Jan 27, 2012 that added the ability to earn Monkey Money for completing Daily Challenges.
An update was released on April 23, 2012 that added the ability to earn Monkey Money by 20% reward instead of original reward (100% reward) for replaying all Tracks.
On April 26, 2012 there was a update for double Monkey Money. Now you get double monkey money every time you win a game, Daily Challenge or Special Missions. You also get double from the treasure chest. This ended April 29, 2012.
An update was released on the July 12, 2012, that allowed for a Treasure Chest Monkey Money reward multiplier.
Double Monkey Money came again from August 25, 2012 to August 27, 2012.
Double Monkey Money came yet again from October 4, 2012 to October 7, 2012.
Double Monkey Money came a fourth time from October 25, 2012 to October 28, 2012.
Double Monkey Money came a fifth time from December 19, 2012 to January 8, 2013.
It looks like an American dollar bill in real life, except they have a monkey (apparently named George Monkeyton) on them.
George Monkeyton is a pun on George Washington, the first president of the United States.
It's much easier to get Monkey Money on BTD5 Deluxe/Steam, as many rewards are twice what they are on the online version in addition to four times the reward for Special Missions.
Speaking of BTD5D, you can get premium upgrades by using Monkey Money instead (Though much more expensive)
In iOS versions of BTD5, the player can get monkey money by opening the chest everyday.
If the player goes to their devices settings, then Date and Time, they can change the date. Changing the date makes the iOS version of TD5 think that it is a new day, so the player can then open the chest multiple times everyday, giving the player "Unlimited Monkey Money."
However if you do this and set the time back to normal the chest cannot be used until the actual "day" comes.