You can get from table PMCO.
Input
The OBJNR value of Order number (You'd get it from AUFK table)
And in field WRTTP value '1'.
Output:
The Plan cost will be available in field WRT03 if your Order date is in June. (WRT01 for April .....WRT12 is for March).
Function modules:
You can explore these FMs
IBAPI_ALM_ORDERCOSTS_READ
PM_WORKORDER_COSTS_MAINTENANCE
PM_WORKORDER_COSTS_LIST