we do the setting for this in copy controls from delivery 
to billing.

pl go to t.code VTFL, select the item which is required to 
abasorb the cost.

there you will find a check box " cummulate cost ". Tick it 
here, this will make the cost of the free item to be 
transferred to the main item. This can also be used even 
incase of BOM's.

First enter the main item in the sales order and then while 
entering the free goods material you will find a column 
called "Higher Level Item" on the sales tab page, you have 
to enter the Item Number of the Main Item (eg: Item 10) in 
this column. Now the system will understand that you are 
referring to the Free Item and will process the Item 
accordingly.

Thanks