Expectancy-value models of motivation have been applied to understanding children's choices in areas such as academics and sports. Here, an expectancy-value paradigm is applied to exercising (defined as engaging in physical activity). The notion of perceived cost is highlighted in particular. Two hundred twenty children in third, fourth, and fifth grades were surveyed on their competence beliefs, perceived importance, interest, and perceived cost of being physically active. Results indicated that perceived cost is empirically distinct from competence beliefs and other types of value, that perceived cost is marginally related to children's self-reported level of physical activity, and children's beliefs and other values are related to their self-reported level of physical activity. Children's perceptions of cost vary depending on grade and gender. Interventions based on these findings are proposed. (Contains 2 notes and 1 table.)