OK, so for the combo approach you make two relationships
You have your existing relationship and the reference field is [Related Rate Card]. I suggest renaming it to be called
[Related Rate Card from drop down]
Then you already have you data entry first called
[Rate Card Direct Entry]
Then make a formula field called [Related Rate Card final]
IF([Rate Card Direct Entry]>0, [Rate Card Direct Entry], [Related Rate Card from drop down])
Then make a new relationship and use that [Related Rate Card final] field on the right side of the relationship. Lookup any values using that relationship and delete any lookup fields from the original relationship and instead us these lookup fields.
