Dear Nischith,
This can be done in standard way kindly do as per below
1. Create statistical condition type for RM Cost in pricing procedure and add with subtotal (Basic + RM Cost).
2. Write routine in tax procedure "Base Type - 362" and write the logic to fetch the value from subtotal instead of base price.
3. Create PO with Basic + RM Cost, tax will be calculated on subtotal (Basic + RM Cost).
4. Do GR and MIRO.
Regards,
Karna J