Cost determines price

In the long run, expenditure (whose value is cost) determines price. This doesn't opposes at all to the theory that, given a production-possibility frontier, value alone determines prices. It is just an assertion that has to be interpreted very carefully and with a lot of subtlety. It means that, in the long run, it is not the concrete (as disconnected from anything else) value ascribed to a good what determines its price but that value in the context of comparison with the rest of goods (general equilibrium). This is how we have a remembrance, a warning, that whenever we make an analysis allowing for enough time as to everything to adjust, we can not forget general equilibrium. Partial equilibrium analysis is just the the first stage of general equilibrium analysis.