I have never used filemaker but it is a relational db so their is most likely either a sql window or graphic sql designer like in acccess.
If there isn't you will have to find out how to join tables in filemaker, then join the items table to the costs table where the expiry date is null.
Simple sql
SELECT a.Item_name, b. Costs
FROM items a, costs b
WHERE a.id = b.Item_id
AND b.expiry_date IS NULL
Sorry I couldn't help with the filemaker syntax, there might be someone on here that knows the product.