Need multiple fields to lookup from a child to a parent record

It might just be me, but it seems like you might need another table.  Basically a joined table that would be your "Project Estimate Line Items"

Then when you add a line item, you pick the product (all the lookups come with it, Price), then they entry the Qty.  And a simple formula gives you the cost per line item.  Then a summary for the total estimate cost.

Not sure the reverse relationship is needed.