Best way to create a Link Table. What you can do is create a concatenated key with ProductItem, CostID and generate a link table.
Your tables would have:
1. ordercosts: Orderline, LinkTabKey, Orderprocessingcosts
2. productioncosts: LinkTabKey, Productioncosts
3. LinkTab: LinkTabKey, ProductItem, CostID
Try first and if need further help, just post a sample data file here.
You can keep join the two tables on the productitem field and rename CosID in the first table to OrderCostId and CostId of the second table as ProductionCostId.
That will keep the two cost types separated but will avoid the circular reference.
CostId as OrderCostId,
CostId as ProductionCostID,
Hope that helps.