Try to pay at or a little below what is being paid out by the bigger buyers like boardsort, etc.. They obviously have their price points so that they still have room to make money, and I believe they adjust what they pay based (somewhat) on spot price of gold. So I would say thats a good start.
But, if you have to know per unit.. I would weigh one, and do the math to figure whats in a pound and what its value is from there.