the calculation of the price is actually made to allow the accountant to double check the calculated price with the printed price on vendors invoice or even with the price from the PO.
Because of that I would not replace this calculated price with the price field from the PO.
Maybe adding the PO price field next to this calculated price could be an idea .
But to answer question. I don't know any standard way just with customizing to make that happen. (an Abaper would probably say that some coding in a user exit is a standard way)