DragonCash can be purchased from the Market for real currency. Like Treats and Gems, you are offered more "incentive" DragonCash when you spend a larger sum of money at one time. The currency and rate changes by level and country.
The most common way to earn DragonCash is to put Dragons in Habitats and collect the money when it fills up. An icon of a filled treasure box will appear when a habitat is one-third full, and will switch to an icon of an overflowing treasure box when it reaches 80% of the habitat capacity.
DragonCash can be collected by tapping on the icon or by tapping on the habitat before enough DragonCash accumulates and tapping the "Collect" button.
DragonCash can also be earned from selling unwanted Dragons, racing in Dragon Track, finishing events in the Colosseum, and tapping on party hats in your Friends' parks.