We study the role of pricing in Differentiated Services (Diff- Serv) networks. We model DiffServ as a priority service, where users are given the freedom to choose the priorities of their traffic, but are charged accordingly. Using a game theoretic framework, we study the case where users choose an allocation of priorities to packets in order to optimize their net benefit. For the case where users with bursty traffic access a single link, we show that there always exists an equilibrium for the corresponding non- cooperative game. Furthermore we show that pricing can be used to provide relative QoS guarantees.