Nicholas Kristof mentions the effectiveness of deworming, based on a study by IPA Research Affiliates, in a discussion of changing attitudes towards foreign aid.
"Take education. Given the problems with school-building programs, donors have turned to other strategies to increase the number of students, and these are often much more cost-effective: (1) Deworm children. This costs about 50 cents per child per year and reduces absenteeism from anemia, sickness and malnutrition. A Kenya study found, in effect, that it is only one twenty-fifth as expensive to increase school attendance by deworming students as by constructing schools."