Does selecting "do not replace existing value" check box for this field make a difference?
If not, you may need to capure the amount in a variable, then use set field to put the value back after duplicating the record.
You do have another potentially serious isssue here. You use Go To Related Records without first checking to see if any related records exist. If you should ever run this script when you have no line item records, the Go To Related Records step will leave the current found set of records on the related records layout unchanged and thus a completely unrelated set of records might be duplicated by this script.
Compare you script to the one used here to see an example of testing for the existance of related records: Duplicating Bill Of Materials (duplicating portal line items)