We address a cloud spot bidding problem for user cost optimization. We propose stochastic optimization model to minimize the expected resource rental cost in the presence of spot price uncertainty. The model is based on the well-known full-information best-choice problem. Based on the model, we derive the strategy for cloud spots bidding. The strategy allows to minimize the expected cost for a spot instance in a specific period of time with quality of service guarantee. Our simulation analysis based on realistic settings clearly demonstrates the advantages of the proposed optimization solutions.