SAP does not standard copy the price from Purchase Requisition to the Purchase Order. It is either the price from contract (if referenced), material master, Inforecord or a previously created PO. To enable SAP to take the price from the PR you should create a function authorization profile in customizing:
1) in SPRO go to Material Management > Purchasing > Authorization Management > Define Function Authorizations for Buyers. Then select Function Authorizations : Purchase Order. Here you can define/restrict what a buyer can do in a PO. Make sure that “Order price adoption” is selected.
2) Also you have to assign the Function Authorization you created to a user. Enter the key as a parameter under the parameter ID EFB in the user master (SU3).
3) Now you are set to go. These settings allow the buyer to select in the PR that the price is taken over in the PO. This is a selection in the valuation tab on item level in ME51N. It is not defaulted!