Decision aids are increasingly used to support doctors and patients in shared health care decision making, yet methods to measure their benefits for economic evaluation have received limited attention. Significant non-health benefits such as improved patient knowledge, experience and satisfaction may accrue through the use of decision aids. These cannot be assessed within the dominant health economic framework of cost utility analysis. The objective of this paper is to propose a new opportunity cost-based method suitable for assessing the benefits of decision aids relative to other interventions in a resource-constrained health care system. A literature review to identify how decision aids have been evaluated found that economic evaluations are limited. Non-health benefits have been measured with knowledge and satisfaction questionnaires, but have not been assessed within a choice-based methodology. The key function of a decision aid is to provide information for a patient to make an informed decision and the displaced activity within the health care system (the opportunity cost) would be physician consultation time. We propose a stated preference consultation time trade-off (CTTO) in which a proportion of a hypothetical 10-minute physician consultation is traded for use of the decision aid by a patient with prior experience of the aid. Conceptually, a patient is trading a substitute source of health care information to maximise their utility of the consultation experience. The CTTO may be reported in consultation minutes or converted to a monetary value using the local cost of physician time. These values can be used, alongside the cost of the decision aid, for economic evaluation. The CTTO is currently being evaluated within a clinical trial of a cataract decision support tool.