The $1.99 button refers to App Store credit/Android Store credit, not City Cash. That means it costs real money to purchase. If your device is an iOS device, top up some iTunes credit and then press the button again to purchase the Banana Farmer for $1.99 real money. If your device is an Android device, top up some Android Store credit and then press the button again to purchase the Banana Farmer.
I am no expert at the Android operating system, but for whichever app store your device supports, fill credit for that particular app store.