In 2007 the US Congress created the priority review voucher program to encourage the development of drugs for neglected diseases. Under the program, the developer of a drug that treats a neglected disease receives both a faster review of the drug by the Food and Drug Administration and a voucher for a faster review of a different drug. The developer can sell the voucher. We estimated the commercial value of the voucher using US sales of new treatments approved in the period 2007–09. A third of the commercial value of a voucher comes from capturing market share from competitors, nearly half from the value of earlier sales because of the expedited review, and less than a quarter from lengthening the time between approval and the launch of a generic competitor. We estimate that if only one priority review voucher is available in a year, it will be worth more than $200 million, but if four vouchers are available, the value could fall below $100 million. Congress should be cautious about expanding the voucher program, because increasing the number of vouchers sharply decreases the expected price. Lower voucher prices could undermine the incentive to develop new medicines for neglected diseases.