A cluster provider provides cloud computing instances that can be rented (e.g., by the hour) and configured to perform cloud computing jobs. Demand for cloud computing services varies over time; enough cloud computing instances exist to satisfy even a peak demand. A varying number of unused cloud computing instances therefore are available at any given time. In order to derive some value from the unused cloud computing instances, the cluster provider offers them for use at a lower rate, with the caveat that they may be recalled at any time. It is in a customer's best interest to take advantage of the lower rate, however, the desire for the lower rate must be balanced against the risk of losing computing resources at any time.