Where does the portal come in with that scenario? The presence of a portal implies a third table.
The Invoices starter solution can be examined as a working example of how this can be set up. There's a table to Products that stores a price for each product. And records in the Invoice data table lookup (copy) the price when a matching record is created in Invoice Data. This is done on a layout for Invoices that has a portal to Invoice Data.
So the first step in your problem is setting up the needed relationships. Do you have relationships in place linking the two tables? What relationship did you set up? What match fields did you use?
It seems that no one knows how to do this or never has done something like this.
What we want to know is (besides of relationships) how to autopopulate a field based in a selection dropdown list made in another field.
I have the same problem. I am inserting a picture to illustrate what I mean so you may help us with this situation.
Hope you can see the image properly.