Distributed systems require declarative access to diverse information sources. One approach to solving this heterogeneous distributed database problem is based on mediator architectures. In these architectures, mediators accept queries from users, process them with respect to wrappers, and return answers. Wrapper provide access to underlying sources. To eeciently process queries, the mediator must optimize the plan used for processing the query. In classical databases, cost-estimate based query optimization is eeective. In a heterogeneous distributed databases, cost-estimate based query optimization is diicult to achieve because the underlying data sources do not export cost information. This paper describes a new method that permits the wrapper programmer to export cost estimates. For the wrapper programmer to describe all cost estimates may be impossible due to lack of information or burdensome due to the amount of information. We ease this responsibility of the wrapper programmer by leveraging the generic cost model of the mediator with speciic cost estimates from the wrappers.