Unreal Engine 4 is free. With source code and sample framework of a game (via github). Only have to pay 5% royalty on games you sell. I believe it comes with a map editor like UDK.
Not an option for me right now due to system resources, but you all might be interested in it.
Not sure what the deal is the UDK/UE3. I don't believe you can get both the source code and a FPS framework for free.