Every time you earn bonus coins in a Match 3 puzzle, even more bonus coins will go into the Bank.
Once the Bank reaches its minimum of 3000 coins, you have the option to purchase it (with real money) to obtain all of the coins inside it.
The Bank can hold up to 5000 coins, but the price to purchase the bank’s contents will never change regardless of how many coins are in it.
The Bank’s coin contents reset back to 0 whenever you pay to open it.