<old answer>Sorry Jainesh - this appears to be costing a manufacturing order - I just want a simple BOM cost</old answer>
Ah, ok - I have found that report. So are you saying I have to run the report on each of my assemblies, and then manually enter it in each item?
That seems bonkers - everytime a component price changes, it means someone first has to run a where-used report, then someone has to re-run the BOM cost report and then finally enter the new costing in the affected assemblies. This is what computers are for!