A system and method is provided for generating and using purchase strategies based on the price, performance, and/or other information related to cloud services to optimize the selection of such services. The purchase strategies may comprehensively describe various cloud services in real-time so that customers may purchase cloud services using up-to-date, real-time information. The purchase strategies may, for example, describe pricing, performance, availability, and/or other attributes of various cloud services. A purchase agent may use the purchase strategies, one or more purchase rules, and/or other information to generate a purchase specification that specifies one or more cloud service instances that should be purchased. The purchase agent may leverage unique properties of spot instances to make favorable purchase decisions. For example, the system may determine bid prices that should be made to obtain certain spot instances.