Supply and demand.

When nobody wants an incredibly common commodity it's worthless. When everybody wants a limited commodity it's expensive. 

Edit:

"Limited" can be either natural or artificial limits on supply.

You can't just artificially inflate the price on lobster because it's harvested by countless independent fishing boats and distributed by countless independent shipping companies and retailers. There's no "lobster cabal" that can set prices -- the prices are determined by the market based on current demand and current supply.

Hence "market prices" for fish and lobster.