The Piggy Bank is a special one-time purchase that grows over time. Every time you finish a mission, gold is added to the Piggy Bank until it's full. When it's full, or you're ready to collect, purchase the Piggy Bank to smash it! You'll get the gold inside instantly. Make sure to buy it when you're ready to get the gold, as it won't grow any more once purchased!
The Piggy Bank will remain until you're ready to purchase it, but you're only allowed to buy a single Piggy Bank per account.