Heterogeneous or federated clusters are a significant special case of grid computing. An important factor in efficiently executing applications on such clusters is the mapping (or scheduling) of application tasks onto cluster nodes. The cost/benefit estimating service aims at facilitating the search for efficient mappings both initially and during the evolution of a computation. This service dynamically evaluates alternative mappings using system and application information. Building the necessary database for use with this service involves profiling applications and running benchmark suites on cluster nodes. Recent data obtained at Sandia National Laboratories, NM, demonstrate the existence of differences in computation and communication speeds between the nodes of even simple homogeneous clusters, as well as the feasibility of using such data for predicting the efficiency of application mappings.