Field relativity

I've recently run into some situations where associating two (or more) CCK fields together would be useful. I know that Node Relativity does this for nodes, and that CCK itself has node reference and user reference field types, but I want to do the same sort of thing between fields within a CCK content type. The Fieldgroups module only allows for organizing similar fields into nice user interfaces, as far as I can tell, and does not deal with relationships between fields.

For example, I have an Invoice content type that has Billable Item as one field and Amount as another. Both of these fields can be multiple (repeating), but there is no way I can see of associating a specific Amount field with a specific Billable Item field (for example, "Setup and installation of Widget Master" and "$500.00"). I might even want to associate a third and fourth field, Unit price, and Num Units, with the other two fields ("$100.00" and "5", respectively). I could then use an SQL snippet or View to print out my invoice.

Before I start protoyping this as a new CCK field type, I'd like to hear from others who might find it useful or from people who have worked around this type of problem. To summarize, I'd like to be able to associate instances of specific fields to each other to form relationship-based data structures within CCK content types.