If you have a single price for the item, you could consider adding it to the row source of the combo / list box.
This wouldn't work though if you had more than one price for the item.
It wouldn't be a smart idea if you have a large numnber of products on the list box, nor is it 'smart' to have more columns than required in the list box
so lets assume that you added the invoiceprice to the rowsource for the combobox then you can pick up the price from the relevant column.
me!invoiceprice = me!productid.column(x) 'where x is the order the invoiceprice is in the row source for the combo box
IMnsHV domain functions may have a good use in VBA, but I haven't found one yet.
Why don't you just in your Table a Product Field and a Price Field for that product. Then when you make your ComboBox and Select the Product, the Price for that product automatically comes up. See the small sample I attached that does just what I'm talking about. At least it seems that is what you are wanting.