How expensive is a college degree? Usually, the answer is based on what students pay in tuition and fees compared to what they earn after graduation. Very little is known about the economic cost of running an electrical engineering program compared to, say, a history department, or the resource consequences of steering more students into these fields. To fill this gap, the authors examine department-level data on expenditures, outputs, and factors of production for undergraduate, graduate, and professional degree programs at nearly 600 four-year institutions across the United States from 2000 to 2017. The analysis compares the instructional costs per student credit hour at more than 8,000 departments in 20 disciplines, including both in-person and online study. The authors establish five new facts about college costs: (1) substantial cost differences across fields of study; (2) most of these patterns can be explained by differences in class size and, to a lesser extent, differences in average faculty pay; (3) cost differences have evolved over time; (4) these trends are explained in part by a growing number of adjunct faculty as well as changes in class size and faculty teaching loads; and (5) online instruction is not a cost-saver. The results underscore that the social return to investment in high-earning fields may be lower than wage premiums suggest, because high-return fields also tend to be more costly to teach. In addition, the work suggests that while differences in production technology enable some departments to take different approaches to cost management, from changing the mix of faculty to increasing class size, online instruction does not have a meaningful association with college costs, at least in its current form. Any one-discipline-fits-all approach to addressing cost escalation in higher education, including moving more classes online, is likely to be ineffective.