There are many factors to consider when determining the cost of concrete driveways. Every job is different so it's difficult to get a good estimate without having the contractor see the job first.
Obviously the size of the driveway is the most important factor. The more concrete needed, the higher the cost. There's a simple calculation for determining the amount of concrete needed. Find the square footage and divide it by 80. This is for 4 inch thick concrete which is the standard for residential installation.
How much preparation work is involved before the concrete gets poured? Is there any digging and dirt removal that needs to be done? Does the dirt need to be hauled away? Is there existing concrete or asphalt that needs to be broken up and hauled away? How much granular fill is needed? Usually only those that do this for a living can look at a job and have a good idea on how much prep work is needed. Another factor to consider includes accessibility. Can the concrete mixer reach all areas to be poured? Are there any overhead power lines too low to allow the trucks to pass? Is there a culvert or bridge that can't support the weight of the trucks? If this is case, then the contractor must use a pump or buggy to place the concrete which can significantly increase the cost of concrete driveways.
What type of finish do you want? Decorative concrete can be double or triple the price of a normal broomed surface. Concrete stamping requires highly skilled labor as well and at least twice the amount of time for placement. Some other factors that will raise the price are reinforcement and temperature. Using steel reinforcement such as re-bar and wire mesh is becoming less common in residential concrete. Most estimates will not include reinforcement unless the contractor sees a special need for it. The cost of concrete driveways also varies with location. As the cost of living goes up, so does the price of any type of construction. A driveway on the west coast may cost three times as much as the same driveway in the Midwest.
When getting estimates for your driveway, don't just go with the lowest bid. As with anything else, you usually get what you pay for. Be sure to ask questions and get some references. And don't be quick to try and talk down the price because you may be talking down the quality of work as well.