Web services composition has gained a considerable momentum as a means to create and streamline business-to-business collaborations within and across organizational boundaries. The composition approach proposed in this paper can enable user to generate a composite web service with minimal execution price dynamically. After modeling Web services with rules and eliminating semantics conflicts among parameters of Web service models, we can compute a deduced network with inputs given by user. Based on the deduced network, a set of composition plans for every output can be obtained with a backward deduction approach. In order to get the composition plan which has the minimal execution price, all the output's composition plan sets should be joined by Cartesian product.