To enjoy the benefits of cloud computing, it is necessary to select the providers whose offer best fits user's needs, carefully evaluating features and pricing. Currently, the choice of a cloud computing vendor is really difficult because it needs to map application requirements, well known to cloud customers, properties of cloud services and service levels offered by providers. Furthermore no negotiation services are available, and options are different in terms of service properties, general conditions and service levels that providers ensure. Service Level Agreements (SLA) use proprietary metrics that make difficult to evaluate properly each offer and to perform a clear comparison. A decision support for the user to choose the best cloud solution that satisfies his/her requirement is desired. In this paper, we present a model for measuring compliance of SLAs by different providers with the user's requirements and for their comparison, in order to broker the best proposal the cloud market. We describe its implementation within cloud agency and discuss preliminary experimental results.