It depends if already some source(info record, Contract etc) for the vendor material combination is existing or not. if some thing is there system will default the price automatically but if nothing is there then it will ask you to enter the price.
you can test it just take a material-vendor for which no info record/contract is there is there and then create PR-PO and make sure u tick the info update in PO under material data tab page, system will ask you for the price enter it and save it. now next time take the same combination again create PR-PO u will see system is not asking for the price as info record is already created because info update tick u did while creating last PO.
SAP has automatic Info Records set when you save a PO for vendor/material. When you make a new PO the system finds the price from the latest PO (same vendor/material). You have to set the authorization to transport the price.
Do you always want to adopt the PR price in PO? Reply and I'll tell you de IMG steps.
MM-Purchaching-Authorization Management-Define function.
Here create a new function (name it 'PO') and tick all boxes (preferable than just reference to PR). Save it and then, in SU3, create a parameter ID 'EFB' with the value = 'PO'.
This will make a new field appear in ME51N... set this field however you like (you can fix it in personal settings for PR's.
Let me know if you have any problem (you can still check on notes for it).