If you are looking to add a 2nd story, and wish to know that your general contractor quotes are market priced, I suggest that you obtain at least 3 offers for the work. I can provide you a short list of contractors if you like.
One additional consideration which can affect the price of your second story, not from a build point of view, but rather from a resale point-of-view, is if that newly added 2nd story creates a nice view from a new master bedroom or new family room, etc. Unobstructed views tend to add to the value of a property, other things being equal.
All the best in your endeavor.