In an empowered Internet with end-to-end Quality of Service (QoS) facilities, it is essential for content servers to minimize reserved network resources to achieve a reduction in transmission cost for the use of QoS. Resource reservation usage charging forces customers to efficiently use network resources. In this paper, we analyze a model that optimizes the total cost when semi-elastic traffic flows are delivered in a client-server scenario. The client uses the proposed analytical model to easily control its buffer occupancy and to determine at any time if it is needed to utilize resource reservation or best-effort transmission mode.