Define matching price fields for each item in your Portal's table. Set them to look up the price from the matching field in your inventory table when an item number is selected/entered.
Add a PriceLevel field formatted with a value list of your price level names (level1, Level 5, etc.)
Make your price field shown in the portal this calculation field:
Case ( PriceLevel = "Level 1" ; PortalTable::Level1Price ;
PriceLevel = "BTG" ; PortalTable::BTGPrice;
PriceLevel = "Level 5" ; PortalTable::Level5Price ;
and so forth to account for each price level.
This is the simplest way I can think of to implement your pricing system. It's also possible to set up a separate related table of prices with one price in each record. You'd then do a look up from this table with a relationship that matches by both Item # and Price Level fields to a specific pricing record in this table. This is more complext to set up, but much more flexible to work with as you are no longer limited to a specific number of price levels for any one product.