That is such a difficult answer to give. The variables involved in a roof deck are huge; the material you will use for the deck, is the existing roof structure adequate or does it need enhancing, what are the requirements for your jurisdiction, what are you going to use it for (that will make a difference in how you build it). Access to the roof both for the construction process and for actual use when you are done.
One thing to remember is that anyone who tells you they can give you a number right away is just plain wrong. They simply wont know enough until they spend a good amount of time exploring all of the variables. The best thing to do is find a reputable contractor who you feel you can trust and who will be open with the process and pricing.