The Banana Research Facility (BRF) is the fourth and final upgrade to the Banana Farm in Bloons TD 4, or the final upgrade for path 1 in BTD5, giving $2000 money every round. It costs $8500 on Easy, $10000 Money on Medium, and $10800 on Hard. In BTD5 it costs $11900 on easy, $14000 on medium, $15120 on hard, and $16800 on Impoppable. It is unlocked at Rank 28 (36 in BTD5). This tower is important to get you far in the game as most towers will be near useless in later levels and the ones that make the most difference (Sun God, Laser Cannon, Tempest Tornado, etc.) will be really expensive, making them a big payoff.
Because there are fewer rounds in Bloons Monkey City, the Banana Research Facility brings $300 per box ($450 as a 4/2) instead of the normal $200, giving a possible total of either $3000 or $4500 per round if all banana boxes are collected. Also, due to the competitive nature of Bloons TD Battles, each box grants 50% more money compared to BTD5. In Bloons TD Battles Mobile the Banana Research Facility also produces 6 boxes per round instead of 5, giving a possible total of $3600 or $5400 per round (presuming all boxes are collected).
The Banana Research Facility is the only upgrade of the Banana Farm to have no open space. There is a grey rectangular prism at the bottom of the facility. There is a large light blue window on the prism. Then there are two pipes on top of the right side of the prism. Finally, there is a cube on the left side of the prism, looking exactly like it. The only difference is that there is a banana sign on top of the cube.
Use this strategy for Medium or Hard. You will need to buy the Banana Farm in the early rounds (ideally before Round 10), and then upgrade it to Banana Plantation as quickly as possible. Then after 5 more rounds, you should try to buy Banana Republic. Then when you get to Round 40+, try to buy the BRF. This upgrade should be bought at or before Round 53 on Medium, or Round 57 on Hard. Let the money roll in!
Getting ten of these in BTD4 will make the player get $20000 per turn and will allow the player to get tons of Sun Gods. (1 each round, in fact, assuming that the player already has a plasma monkey at the end of each round)