Calculate the cost of concrete based on a per cubic yard price. Pretty straight forward and sounds simple enough. But this will not always be the case. Depending on how and when you are pouring your concrete can have a drastic effect on your cost, and that great quote you got from the concrete supplier can end up being higher. On a hot day, using a line pump, you will have the additional cost of having to use smaller gravel, and you might be looking at a few concrete additives to make it possible to place and finish the job. In the winter, if it gets cold enough, they may have to use hot water in the concrete mix and use additional admixtures to get the job done, all adding to the cost.
If you are not doing the work yourself, talk to the concrete finisher beforehand to find out if he will be needing any admixtures. Not like it will help much................... If you are pouring yourself, it is always a good plan to have a concrete vibrator on site which will add to the overall cost of the concrete. And then of course smaller jobs can turn out much more expensive for the cost of concrete, than larger jobs. You could be looking at an added cost for small loads, a delivery charge, and you are more likely to be charged for waiting time if it takes longer to empty the truck than the allotted time.