A feasibility study of a new plant or even of a revamped one bases the forecast for incomes and outcomes on a discounting back approach. This means that both prices and costs of commodities are assumed constant for long periods. The paper tries to solve the “discounting back” problem that sees a coming apart between the dynamics of real market prices/costs (subject to fluctuations, volatility, and the “supply and demand” law) and the constant prices/costs assumed in conventional feasibility studies. The manuscript presents and discusses a methodology to model the time evolution of prices and costs of commodities.