There is currently debate on whether to expand the US physician workforce and how to finance it. Whether an expansion is achieved through development of new training programs or expansion of current programs, the financing options seem limited. The literature lacks information on the marginal costs of training physicians, however credible marginal cost estimates are critical to get funding for program expansion. In this study we determine annual direct marginal costs of financing physician training in the US, using 2000-2007 financial and enrollment data from the association of medical colleges and Federal and State agency sources. We found marginal costs ranged from $190,000-250,000 a year per allopathic student, $55,000-83,000 per year per osteopathic student, and $75,000-106,000 a year per resident. Our estimates remained robust even when we used alternative costing methods.