There's much debate of this.
If my sources are correct two spawn during the Lord Oaks spawn. People have been looting them.
I saw a thread on the Stratics rares forum that most people were saying around 500K I think but one guy says he can get much more (like a couple million)

Perhaps since Hemisphere has been visiting our boards he might be able to shed some light on this. He's one of the most knowledgeable rares owners around.