Last Cost Prices (Standard)

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Hi all, 

I have seen a couple of similar queries but none have been able to answer my question - 

I have used Price Lists to update prices for a  supplier and successfully imported them, however, when entering the items into a PO they continue to use the Last Cost Price (in the item set up) rather than the new supplier price list that I have imported. 

I have assumed that the Supplier price list purchase price may be used by Sage 50 for profit reasons, rather than for entering PO's?

Is there a war around this? We changed them manually however each time we book in a PO a an old price, the price in the Item Code reverts back to the old price.

I would like o be able to update all item prices inline with manufacturer increase, then continue to use them without repeatedly having to amend them back after they have been booked in at an old price. 

Thanks in advance