Background: Children growing up in poverty tend to perform worse in school than their more economically advantaged peers. Aims: The current study integrates an educational theory of motivation and an evolutionary theory of life history strategies to examine how economic disadvantage predicts children's mathematics achievement through their academic beliefs and values. Sample: Participants were 1,536 students (M[subscript age] = 12.88) in a large metropolitan city in the United States. Methods: Economic disadvantage was assessed via school reports of the student being eligible to receive free or reduced-price lunch during the 2014-2015 school year (i.e., at or below 185% of the federal poverty line). Students reported on their perceived interest, usefulness, and cost of learning mathematics during the first half of the 2015-2016 school year (August to December). Mathematics achievement for both school years was assessed via school reports of mathematics grades. Results: Children receiving free or reduced-price lunch showed higher perceived cost of learning mathematics, and this in turn predicted changes in mathematics achievement over time, indirect effect = -0.57, 95% CI (-0.97, -0.23). However, neither interest nor perceived usefulness or ability in mathematics mediated the association between economic disadvantage and changes in mathematics achievement. Conclusions: Results underscore the potential for interventions to target students' perceived cost of learning as a way to increase school engagement, particularly among disadvantaged students.