Associate additional costs to Assemblies?

Hi,

I'm overhauling our inventory setup, specifically our assembly/sub-assembly topology and configuration.  One requirement is to associate fixed service costs with our assembled goods.  For example, we use an assembly service to put some widgets together, let's say the assembly service vendor charges $10 per widget.  When we enter a new Assembly Build Transaction we want to associate that $10 charge with the assembly so that our costing will be accurate.

BTW, don't be fooled... I don't really know what I'm talking about, accounting is not my thing.

So, I thought I could add the Service For Purchase item that we use on the Purchase Order to the Assembly record.  First thing I noticed is that Vendor-specific pricing is not pulled in (it would be great if a Vendor column would support this!).  Moving on I tried setting the "Purchase Price" field and it did indeed pull the value into the Assembly Build.  However, when I saved the Transaction I was surprised that the Projected Value did NOT reflect the Service For Purchase.  Is it because it's a Service For