The Bank is a feature, found in the Menu. In the Bank, you can exchange your Bux () for coins () to buy airports. The more you exchange for , the higher the Exchange Rate, as seen below. You cannot exhange your for , though.
If you are planning to exchange your Bux to get more coins , it would be strategic to save up the most as possible because the Exchange Rate grows higher when you Exchange more .
The formula used to determine how many coins you get per Bux is: 500 + (number of / 2). If the number of is odd, the resulting fraction is rounded down.
While converting to , you MUST make sure your value never excedes 2,147,483,647. If it does, the data will excede the maximum in-game capacity and reset to 0.
To get the TOTAL number of coins for "number of ", simply multiply the " per " that you just calculated, by your original "number of ".