Is parecon’s price determined according to labor value theory or marginal utility theory?
Prices for coarse categories of goods and services are set by the Iteration Facilitation Board (IFB) during Annual Planning according to proposed supply and demand. You can think of it as:
price_x = some_function(supply_x - demand_x)
where the function is such that excess supply (demand) results in an incremental decrease (increase) of price.
That is what happens. Whether that is justified according to a labour theory of value or a marginal utility theory, or some other model, is another matter. As it happens, expositions of ParEcon have tended to be couched in marginalist terms (utility maximization, equating revenues and costs at the margin, general equilibrium prices).