Calc ? - Return prior year Cost/Unit for the last received purchase order in the prior year

Trying to do this in Tableau Desktop. Solving for Prior Year Last Price Paid. Sample workbook attached. Easier to do in prep but unfortunately that's not an option for me.

I'm trying to return the last price per unit (distinct value) for the same Site and Item from the Prior Year. See screenshot below for a visual example of what I'm trying to solve for. Thanks in advance

Another twist is I do need a tie-breaker also in the event the Site & Item have a PO with the same order date and a different Cost/Unit. The tie-breaker should be to take a max of the two (just to make this a bit more complicated)