Currently process algebras have been effectively used for describing and reasoning web services and their composition, but they are lack of cost modeling and analyzing capability and can't support web services composition with optimal cost. This paper proposes a process algebra called PPA (Priced Process Algebra) basing on CCS (Calculus of Communicating Systems) [1]. We discuss the syntax and semantics of PPA extended with priced information and provide an algorithm to support web services composition with optimal cost.