We have a similar challenge and when we implemented X3, I recall that our consultants said there really wasn't a good option for it. We got around it in a very poor manner (different suppliers for each geography, since it fulfilled from different vendor facilities). However, if you could do this based on one of these fields, it would work. I am not expect on this topic and also posting so I can see other replies!

--Carrier or Incoterms listed on the PO

--Purchase or Receiving Site  ((this might be feasible for you if your prices are receiving site specific-- it works for us on some scenarios))

--Sales site (for direct orders)

Thanks for the reply. I was afraid of that!

Have you tried using a purchase price list that has the supplier, the site you are ordering from and then the purchase price?