Drop Ship PO's - where is the cost pulled from?

We are just now trying to use Drop Ship Po's.   Trying to determine which field the COG is pulled from for the item when the SO is invoiced.    I'm assuming it's a fixed field in the item, just not sure which one.  Also, what happens if you source an item from multiple vendors and the cost is different?  We pay commission on margin....

Thank you!

Ann