Grid resource management plays an important role while enabling the sharing and coordinating of resources in Grid computing environments. Resource reservation is an important part of the Grid resource management. An advance reservation is a scheduling object which reserves a group of resources for a particular timeframe for access only by a specified entity or group of entities. Combining resource reservation with computing economics, we propose a mathematic model which can determine resource booking quantity. In this model, taking economic interest and reputation index of service supplier into account, an optimization problem of two targets under hypothesis is simplified in reason. Through numeric calculation and analysis, we show there is an optimal point for the advance booking of resource.