Although Zombie Farm is free to download and play, there are also several in-app purchases that have actually raised Zombie Farm up to the near top of the highest earning apps in the App store.
BrainsBrains are so rare, you also have the option of buying them through an in-app purchase. Note that Brains have a Weekend Discount, if you wish to buy Brains you can get them at a lower price (20% Off) on Friday, Saturday and Sunday.
You can buy Brains in the following quantities:
In Version 0.88 there was 100 brains promotion with pet brain when you purchase
Nine of the Farmer Heads give you extra abilities. Each individual farmer costs $1.99 USD, 1.59 € or £1.50.
Read more about Farmer Heads.