Hi
If user is changing price in VF01 screen then system dont take it as changes and system wont maintain its log. If user changes the price in VF02 screen then you can check in VF03 > Environment > Changes or in table CDPOS or AUT10.
For your case you can validate VF01's price with VA01 price. I mean you can check billing PR00 price with your sale order PR00 price by writing a code in routine.
Thank$