Systems and methods permit customers to purchase resource instances on a spot purchasing mode basis, which permits a customer to specify a maximum price that the customer is willing to pay for one or more resource instances (e.g., virtual machine instances running on physical servers). Further, the resource instances purchased under the spot purchasing mode can be guaranteed to run for at least a specified period of time, even if the current variable spot cost value is exceeds the original bid price. Such instances run for defined blocks of time which may be selectable by the customer, thereby providing the customer some level of assurance that the instances will not terminate prematurely due to spot cost value fluctuations. Instances are assigned into a spot block instance pool from other types of pool based on the estimated excess capacity of such other types of pools. The estimate of excess capacity is based on statistical analysis of the usage of the instances.