By federating virtualized computing and network resources one can significantly increase their value thanks to gains from statistical multiplexing and increases in resource diversity (more distinct locations, technologies, etc.). Successful federation depends upon resource providers being able to agree on policies: how to share the profit generated by external customers and/or how to allocate the resources contributed by the federation participants to their affiliated users. This paper's main contribution is a method that enables organizers of a federation to evaluate the relative importance of the resources contributed by each participant. We build on coalitional game theory concepts and formulate a generic economic model of federation that captures the notion of diversity, which is relevant for a variety of overlay services, and notably the networking research experiments that are running today on PlanetLab. Based on this model, we propose the Shapley value as a means for participants to share the value of federation. We show how this approach can help in the design of policies that encourage infrastructure owners to federate.