1.) Find an item. If you want more brains find an item that costs brain, if you want money find one that costs money, if you want experience then use one that gives experience, but you can use the experience way with either brains or money way.

2.) Write down the item you chose. I chose the item Evergreen tree.
2 Brains
+113xp
+4 Life

3.) Open up iFile and go to the Zombie Farm app
/var/mobile/Applications/Zombie Farm/ZombieFarm.app
Then open up the file called Market.Plist with the property list viewer.

4.) Find your item. My item is number 52.
Brains Needed will be On, so change the cost to a negative value. I will change it to -50,000 and I will also change the Experience to 1,000. Warning! Setting the Experience given will reward you with Negative Experience so be warned.

Go back into your Zombie Farm app and buy the item you chose, or use mine. You would get 50,000 brains and 1,000 xp.

Enjoy!