Currently, you assign parent items to a recipe, and then when you run the recipe (do the build) you choose which sub you want to associate with the recipe.

Herbo is interested in assigning particular subs to the recipe itself, as they are trying to match the cost in the recipe itself to the cost they have accumulated on a spreadsheet. 

The question Steve asked is if they can wait to determine the cost until they run the recipe? Also, by assigning specific subs, this means you will have to change the recipe every time you use it, as you will have depleted the sub associated to the recipe.