FAM - Anyone know how the Original Cost is derived?

I'm trying to do some scripting to recreate the FAM record creation process, but I can't tell how Asset Proposal manages to fill the Original Cost (custrecord_propassetcost) field. Where does this value come from?

Is it simply taking the average cost of the item?

I've tried searching through the various instances of custrecord_propassetcost in the scripts of the bundle, but that gave me no insight.

If I receive the items from a vendor, I get that it has a value on that PO, Item Receipt or Payment. But if I create a transfer order to move those inventory items to another location I can then generate an Asset Proposal from that transfer order on which the items will have no reference back to the original PO, so how does it still know the Original Cost for the items? I can only assume it's coming through the GL impact, or is it something else?