Monkey Bank is the 3rd upgrade on Path 2 for Banana Farm in Bloons TD 5. Its costs $3570 on Easy, $4200 on Medium, $4535 on Hard and $5040 on Impoppable. When a Banana Farm gets upgraded to Monkey Bank, the farm becomes a bank controlled by monkeys. The Monkey Bank will no longer drop bananas, but instead generates money throughout the round, which is placed in your "bank account", separate from that of other Monkey Banks. It generates $450 per round which can be upgraded to $490 and $630 with the More Bananas and Banana Plantation upgrades, respectively. You may withdraw this fund at any time and the bank will hold up to $5,000. In Bloons Monkey City, the capacities and generation is increased by 50%.
Be aware that selling a Monkey Bank will not withdraw its cash, so before selling any Monkey Bank, please withdraw the cash from such Monkey Banks.