If you do something and what you do costs much more than the market value of its output, make sure it's not your job. Hobbies usually fall in this category.
Let's forget the sunk cost for a minute. What happens if the price per item is less than the variable cost per item? Is that "unfair"? No, that's precisely how firms go out of business, when no one wants to pay (i.e. there is no demand) for the products they sell.
This is not an argument for plagiarism / piracy. Capitalist markets do need strong property ownership laws and enforcement.
In the same way that a liquid betting market, the price is a reasonable estimate for probability, in a any liquid market, the price is a reasonable estimate of value.
Is it not a tautology? Is there a definition of value that is substantially different from "the equilibrium price in a perfect market"?