Hi Brian,
when you say...

"if adjacent cell value (column G part number) is found in column A
(list of all part numbers)"
does this mean 'found anywhere in column A', or does it mean 'found in
the same row of column A' ?

Because you have called column A a 'List of all part numbers' I have
assumed you mean 'found anywhere in column A'.

If my interpretation of your question is correct then I think VLOOKUP
is going to have to be a part of the solution. I don't think that
SUMPRODUCT can work like VLOOKUP ((but of course I could be wrong,
amazing things have been achieved using SUMPRODUCT).

If you add the following formula to a spare column, say column H...

=ISNUMBER(VLOOKUP(G4,A$4:A$58521,1,FALSE))
then fill down to row 58521

then use...

=SUMPRODUCT((E4:E58521=1)*H4:H58521*C4:C58521) in your Sum cell

you should get the sum you are after if my interpretation of your
question is correct.

Ken Johnson