Reference multiple and duplicate part cost in one sheet with costs from another sheet

Hi, please help a newbie!
I have a "Bill of Materials" sheet with multiple "builds" consisting of part numbers and costs used to create those builds.
I have a second sheet of "Parts Cost", where the cost is entered.
How do I associate all the part numbers on the "Bill of Materials" with the cost for that part, on the "Parts Cost"?
Note, some of the part numbers in the Bill of Materials are duplicated multiple times.
Basically I want to be able to change the cost of a part in the "Parts Cost" sheet and automatically update all instances of that part cost in the "Bill of Materials" sheet.
Hope that makes sense.

Perfect, many thanks.
Is there a simple way to show a blank cell rather than "#N/A" when the cell containg the part number is also blank? If not I can delete each instance seperately.
Thanks again.

Brilliant, It's like black magic to me but... case solved!
Thanks again.