The number of population founders and the size of the population are two important variables in determining how much gene diversity can be retained in a population. A model is developed to determine the most cost-effective balance of those two variables, based on comparing the marginal costs of increasing the number of founders vs. increasing the population size. Marginal costs, in this case, are the costs of increasing the number of founders or the population size by one animal. For a goal of retaining 90% gene diversity for 10–15 generations (approximately 100 years), the current recommendation of 20 effective founders is very close to the least-cost solution when the ratio of the marginal costs is equal to one. However, when the ratio is greater or less than one that number can change considerably. Zoo Biol 29:638–646, 2010. © 2010 Wiley-Liss, Inc.