Access control, a task of managing permission or denial to the use of particular resources by particular entities, is achieved by credentials such as passwords and physical keys. Although security of access control has extensively been studied, researchers have not paid much attention to comparing costs of the systems with different credentials. This paper provides a general model of doorkey management systems for access control, where a door represents a resource to control access and a key does a credential. We show door-key management systems based on smart cards, biometrics, metal keys, and passwords will well fit to our model. Then, we introduce management costs to the proposed model. Finding the best door-key management system in terms of minimum management cost is a hard task, since different systems have different restrictions. We present a general algorithm that computes the minimum cost of doorkey management systems with different types of keys.