Ajay,
FG should be defined as Mixed Costed (not SFG).
Once you define FG as mixed costed you can create different production versions for FG based on supplying plant of SFG (one production version per plant supplying SFG).
Then you specify mixing ratio for FG (% for each production version).
In costing system will calculate standard cost per each production version and then will use mixing ratios to come up with "weighted average" standard cost for FG.
Big Choi,
Please correct me if I MIXED 
something up.
Cheers,
Szymon