Is your custom sandwich the product? http://www.alfo.com.au/index.php?main_page=product_info&cPath=25_1&products_id=1
Would anyone want pickled gherkins on plain white bread? Don't answer that ... :D
I've never seen a minimum-price box. What you might do is simplify your pricing. Set the basic price at $4.50 and include one meat or cheese. Set the more expensive items (salmon) as a separate attribute with a separate charge.
For instance, you could replace the current meat/fish/cheese/other with a "select up to 2 of the following" attribute and then offer checkboxes for the meats, tuna, other fillings, and cheese except for Boccocini and Brie.
Then you could set an "add the following for an additional charge" box and list the salmon and expensive cheeses. with an additional fee set for each attribute.
There are downsides to this method. If someone selected chicken breast and cream cheese, the $4.50 flat charge would be less than they would have paid individually. Even the cheapest smoked ham & tasty cheese sandwich would cost $4.90 individually. Maybe you could set the flat price at $5.50 or $6 to be kind of in-the-middle of the meat and cheese range.
As an alternative, you could simply add to the description "MINIMUM CHARGE $4.50 PER SANDWICH" and be prepared to telephone people who order a sandwich costing less.
I'd also make the toasted/not toasted a radio button and move it up so it's right under the "bread" dropdown box.