We provide in this paper the first steps towards the quantification of the impacts of cyber attacks on the power grid. We present a review of key-issues on cyber security of power systems, and show the main challenges as well as complicating factors. In order to do the quantification, we propose the application of a conceptual four-layer framework that represents the physical, communication/control, market levels of the electricity infrastructure, and a cyber security investment layer. We characterize each layer and discuss the relationship among them. We focus on quantify the impacts that cyber attacks can have on the market layer using the system social welfare as the main metric. We use a small system to illustrate the application of our framework on the evaluation of investment alternatives on cyber security.