Look ups require relationships. What relationships have you set up?
What I see here would seem to require 4 different table occurrences of the "paper database" in order for you to look up four different values from four different records in the "paper database" into one record of the table where you have defined this calculation.
There are simpler ways to set this up--such as setting up a set of 4 records, one for each of your four paper cost values that you want to look up for the same job or invoice. You can then set up one relationship from this table to the table of paper costs.