Table relationship dropbox, default value based on field

That is a confusing question. Not sure what drop box has to do with anything here, and certainly, Quick Base does not �provide a drop box�. At least not Dropbox the company where you store files.

Can you take a step back and describe your use case and we will find you a solution?

I'll go through the steps that I did.  

I have an app with 2 tables, one with static data with mail costings and one with dynamic data where we key customer information and provide quotes.

I created a relationship between them based on many quotes to one cost line.

QuickBase then added additional fields in my quotes table with the drop down of the field I want to use as a reference.  

In the quote process we build the reference up based on weight, volume etc and this will match a value in the costs table.

I have the quote sting built in a field.

The dropdown in the form works and pulls back the correct cost, but instead of using the dropdown manually I would like to push a value from a field which has the correct string to use.  In SQL terms this would be 

SELECT cost1, cost2 FROM costs WHERE costs.ref = quote.ref AND quote.ref = "mystring"

The dropdown contains the costs.ref values.

Is it unusual for QuickBase as part of the relationship build that additional fields are created? 

OK, now I understand.  When you said dropbox, you meant in Quick Base terminology a drop down field to make a choice.

So the way that this is done is that in your rates table, you will need to have a Key field which is the concatenation of the unique values which define the rate.   The low tech way to do this is to have a formula field called [Rate Key formula] to calculate that string, perhaps like,

List ("-",[Zone],[weight floor], [Volume floor])

and then on the form have a form rule that says when the record is saved, change the value of a text field called [rate Key] to the value of [Rate Key formula]

Then on the child side you create the a similar formula field as use that as the reference field in a relationship to the Rates Table to automatically pull in the correct rate.