I would assume you have a lookup table in the workbook where you have a row
with the item identifier (from the dropdown) in the left most column and the
price next to it. then you would use Vlookup
Assume
the lookup table is in a sheet named DataSheetName with the identifiers in
column A and the prices in column B.
the dropdown is in B2 of another sheet (Invoice sheet)
in C2 of that sheet as an example
=if(B2="","",Vlookup(B2,DataSheetName!A:B,2,false))