This study examined the cost effectiveness of a 50 lesson mathematics intervention program focused on whole number concepts for at-risk kindergarten students, ROOTS. The study utilized a randomized block design with 1,251 at-risk students within 138 classrooms randomly assigned to one of two active treatment conditions (small groups of either two or five students) or control condition. Proximal and distal measures were collected in the fall (pretest) and spring (posttest). The costs per group per effect-size unit change varied from $216 to $736 depending on differing district scenarios and group size. The cost-effectiveness ratios per student varied from $267 to $3,201 depending on district scenario, group size, and the measure. Implications for conducting cost effectiveness evaluations and public policy are discussed. [This article was published in "School Psychology Review" v40 n4 p359-373 2020 (EJ1282375).]