I had the flat concrete roof of my home replaced in 2006, but I'm in Mexico, so my pricing is unlikely to help you. The first step is to make sure your structure offers adequate support, and that requires a professional assessment. Cost will be affected by engineering involved, thickness of the slab, techniques used, and extra supports as required. Also incorporation of any electrical boxes and cabling. There are likely to be building codes for your area that must be met. Depending on climate, you'll probably need insulation. Here's one link. Just google "concrete roof construction" for more.
http://home.howstuffworks.com/home-improvement/construction/materials/concrete-roofs.htm