ME52N field is not editable.
The field is set as "Display" in relevant "Field Selection Key" in customizing (T-code OLME > Purchase Requisition > Define Screen Layout at Document Level). Please refer to SAP Note 30316 for more detailed information.
Field "Purchase order price" (EBAN-BPUEB) is set in the Purchase Requisition (PR). (Can be checked via T-code SE16).
Related notes / content:
1910199 ME52N : Field is not editable
30316 Problems with fields that are not ready for input in purch.