Context-aware applications are becoming commonplace in everyday life thanks to recent advances in mobile devices, sensor capabilities and wireless communication networks. However, making the manipulation of context information efficient is still an important issue. Taking into account the quality of context data becomes a corner stone of an efficient context management. In this paper, we argue for a fine-grain QoC reasoning. We build on existing application examples and implement one of them with the COSMOS framework. Our approach allows for a better control of the behavior of pervasive applications