The costs never approach zero
How much person/hours and commodities does it take you to "create" a tomato? When that becomes zero (and pigs fly), then the cost becomes zero.
Alternatively, how much are you willing to pay someone else to hand you a tomato, so that you don't have to invest those person/hours and commodities? If this ever becomes zero then either we will all become backyard farmers, or tomatoes will die out as a species.