Thank you Guys, for your helpful inputs.
I only used CONVERT_AMOUNT_TO_CURRENCY FM and it gave correct results.
There are several approaches for this problem-
1. Pass input amount to FM in character format with exact decimal format taking the same from TCURX table. Then the output cost returned is absolutely correct. We would need to firstly use BAPI to convert cost to external format as per currency (BAPI_CONVERT_COST_EXTERN_9.. etc are available)
2. Or we can just pass cost (input/output) parameters in standard SAP way - (Type P Length 12 decimals 2) to the FM and after getting output. Write both Input and Output Costs after currency conversion using WRITE... TO.. CURRENCY syntax. No need of BAPI to convert costs based on currency before passing to FM
Thanks and Best Regards.