In the planning stages of a clinical trial, investigators and funding agencies must make decisions which affect the conduct of those trials. One such decision concerns the choice of the minimum difference in outcomes between experimental and control therapies which would be considered clinically important, a variable which is used to calculate the sample size requirement. This choice is often determined by arbitrary rules-of-thumb. In order to make explicit the resource implications of such arbitrary choices this paper describes a cost-effectiveness model which determines the consequences (in resource allocation terms) of attempting to demonstrate smaller differences in outcomes. Incorporating economic principles into the decision making process in planning clinical trials may be helpful in allowing investigators and funding agencies to set priorities when allocating funds across trials competing for a fixed research budget.