Why would I need the individual cost of the good? That number is irrelevant. The magnitude of the marginal cost is what matters, but only relative to the marginal revenue, the incremental revenue from selling the unit, which depends on the demand of individuals. So, even with marginal cost equal to zero you can still get a positive price because you will never produce until fixed costs are equal to zero. You will produce until the last consumer’s contribution to revenue is equal to the marginal cost. That does not mean that prices are equal to zero. They are always still positive.
Just because somethings costs nothing in marginal costs does not present a great challenge to markets.
