Make and Buy items

Hi

 on 11.5.10.2

We currently have some items set-up as make and buy, however we struggle with costing on these.  We have a couple of scenarios.

 Example 1: we make item ABC, and one of the components is DEF.  50% of DEF is maufactured in house, and 50% purchased from a subcontractor.  Since item DEF can have only one cost in the system, which do we use?  If we use an alternate BOM for ABC, and create a DEF-make and DEF-buy items, which cost do we use for ABC? Primary or Alternate? Seems like any way we go we have variances?

Example 2: ABC is fined item that is 50 % made in-house and 50% purchased. How do we cost these?