Do you earn interest on your bank balance at the end of each wave? I know I get some amount of money after completing each wave, but I'm not sure if it's a fixed amount or variable, whether or not it increases as the game progresses or if it's tied to clearing a wave without letting any bloons through.
In BTD5 you get $99 plus the number of the round, e.g. 100 for round 1, 101 for round 2 ...
Monkey Bank: $3570/$4200/$4535
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. Creates 450 money per round plus 10% interest. You may withdraw this fund at any time, and the bank will hold up to $5000. This will also lock the purchase of Banana Republic or Banana Research Facility for this tower, however. Purchasing More Bananas/Banana Plantation will make the bank generate slightly more money (+ $40/$180 per round).
Banana Investments Advisory: $4675/$5500/$5940
Increased money generated to 1000 per round, increases interest to 20%, and increases the maximum held at a time to $20000 per round. Purchasing More Bananas/Banana Plantation will make the bank generate slightly more money (+ $40/$180 per round). This upgrade is unlocked at rank 36.
You earn 99 dollars + what round you are on at the end of each level. SOURCE
So at the end of level 1, you'd earn 100 dollars. At the end of level 2, you'd earn 101 dollars.