For every new star you earn on any level, you will receive coins. These go into your Piggy Bank.
Once you have collected at least 250 coins, you can either smash the Piggy Bank or keep playing to collect up to 500 coins.
The price for smashing the Piggy Bank will not change, no matter how many coins you have collected.
However, you have only seven days to smash your Piggy Bank. Once the countdown has run out, the amount of coins will reset.