The labor theory theory employed by many anti-capitalists generally hinges on the Marxian unit of account called socially necessary abstract labor time - where abstract means average - because fuck Capital Volume Three's even worse quagmire of prices of production.
This is fun and short;
The following video deals with the Marxist rebuttal to the, admittedly half-baked, mud pie criticism. He points out here that it is only after a good is sold that one can begin looking back through time to try to figure out how many labor-points went in to it;
This one's more lighthearted. Two t-shirts employ the same SNALT as each other but seel for different prices. One features Che Guevara, the other Milton Friedman. Different strokes - and therefore different prices - for different folks;
Aaaaaand a more technical exposition of marginalism;