Price is the quantification of the energetic and material investment necessary to initiate or sustain a physical process, particularly within the context of nuclear reactions and energy systems. This concept investigates the efficiency of energy conversion, the balance between energy input and output, and the consumption of specific physical resources (e.g., fuel isotopes). Key characteristics include its measurement in units of energy or mass, and its use as a metric for evaluating system performance and viability. Its significance lies in enabling the assessment of net energy gain, optimizing system design for efficiency, and determining the feasibility and sustainability of energy production technologies.