Typical Invoicing systems have this structure. Look Familiar?
I'm guessing that LineItems in this example might be SALES in your database.
You have two options here, you can use looked up values to copy all 6 of the prices into separate fields in the sales table and then your case function can be defined in the Sales table to select the appropriate price or you can modify the relationship to include a match by the second pair of fields so that the correct price is looked up when both a code and a pricelist name is entered/selected in your sales table.
With this second option, modify your database design to look like this:
LineItems::Code = PriceList::Code AND
LineItems::pricelist name = PriceLists::pricelist name
Products::Code = PriceList::Code
Perfect! That was very helpful, thx a lot