Though there is an expanding field of research on public prekindergarten, there is a relatively little comprehensive investigation into what public prekindergarten costs. We address some of the absences in the literature by analyzing public-sector expenditures for the city of Boston's public prekindergarten program, a particularly high-quality program. We identify the different components of the Boston program and explore how they translate into costs. We also examine how costs change over time, how costs are distributed across different levels of the education system, and how costs can be adjusted in order to calculate nationally representative estimates. Our findings are relevant for localities interested in learning from existing prekindergarten programs. Our findings are relevant for localities interested in learning from existing prekindergarten programs and can also serve as an example of cost analysis in education research. [This article was published in "Journal of Research on Educational Effectiveness" (EJ1275424).]