Yes it does cost to keep your A/C on, but not as much as most people think!
There has been articles in the press about not using your A/C will save you money on fuel, this is correct, but there are various factors that come into play.
If you are driving a long distance and your A/C is on, the cost to keep it running will be negligible, as once the interior temperature has been decreased it will not take much energy to maintain at the desired temperature, and travelling with your windows down instead will cost more due to drag factor. It will also be less comfortable and the air quailty will be a lot worse.
If you are doing a short 10 minute journey and the car is hot, the A/C will have to work hard to bring the temperature down quickly and you will notice your fuel computer mpg drop whilst it is trying to cool the car down. But, once the temperature is reached the A/C system will throttle back and you will hardly notice any change in mpg.
In June 2003 th UK Department of Transport sponsored some research into mpg done by BTA/IRTE and they found that trucks that had their A/C turned off, but had a window left open increase mpg by 7%. Also, a study conducted by the Society of Automotive Engineers (SAE) proved that it is best to have A/C on than to leave windows opened, please look at full report by Clicking Here.
Also using you A/C will clean the air in your car and dehumidify the air and make it a very comfortable micro climate at a comfortable temperature. This has got to be better than driving with the windows down, filling your car with noise and car exhaust fumes.
In the long run it will cost a little more to keep you A/C on, but using it regulary will keep it working in peak condition and will prevent any expensive A/C repair costs, as most A/C problems are due to lack of use!
In life we pay to have comfort and luxury, so why compromise on the thought of saving a few pounds, which may cost more in the future in expensive A/C repairs due to lack of use.