"World Bank researchers Deon Filmer and Lant Pritchett estimate that the return on spending on instructional materials in education is up to fourteen times higher than the return on spending on physical facilities, but donors continue to favor more observable buildings over less observable textbooks."

William Easterly, The White Man's Burden: Why the West's Efforts to Aid the Rest Have Done So Much Ill and So Little Good, p. 190.

Even if this is over-estimated by a factor of ten, a 40% difference in ROI is enormous.