Bills of Materials

Hey there, can anyone tell me what´s about those Bills of Materials ? I want to use it, but still don´t get it. I want to create an item which includes several other items. e.g. we sell an McDonalds combo. Burger an a coke. The Price of the main item is 10 bucks. Into this “combo item” I include the burger and the coke. Burger is 7 $ on its own, coke 5 $. If i put the “combo item” in an offer it´s 10$, combo price. If i “open” the item, the burger and the coke will appear in the offer and the price is now 12$. What I want is : To book the combo into financials, 10$ and to book the 2 items out of the inventory. I hope this was not to vage, I would appreciate any kind of help.