Dynamic choice which trigger numeric value change?

Does anyone know how to have a numeric value pulled as a result of a dynamic choice please?

I have a dynamic choice field called 'Stock Parts CABLING' which pulls a list of parts from a table called 'Stock Parts' into another table called 'Parts'.

In 'Stock Parts' each record has an RRP, which I would also like to be able to pull into my 'Parts' table depending on the choice selected. I have code set up to do this with a regular choice field, but the dynamic one works differently, and so I can't figure it out.

How do I do this with the dynamic choice field please? It would be much easier this way because I can add parts and adjust pricing in one table and have the dynamic list automatically pull the updated results, rather than having to edit code each time.