Has anyone ran into this issue before?

The only way you might accomplish this is by adding a button in BOM Maintenance that launches a script to calc and display the cost (basically to mimic what the Costed BOM Report does).  The cost of a bill is obviously based on the cost of the components going into the bill, which depends on the valuation method of the component items.  The script would have to fetch the valuation method of each component item and then go through the costing hierarchy for each one just like the software does.