There is basically two different ways to clean air ducts, the Air Sweep method and Roto-Brush method.  Albeit, the roto-brush method is a more thorough cleaning, it is much more time consuming and therefore costs much more.  The air sweep method usually takes about an hour or so, compared to the roto-brush method which can take up to several hours. 

Most air duct cleaning companies charge around $150-200 for a complete furnace and air duct cleaning without any hidden costs using the air sweep method.  For a roto-brush cleaning you could be looking at anywhere between $400-1000 depending on how many registers are in the home. 

The difference between the two is that instead of using just air pressure to dislodge the dust and debris, the roto-brush method actually uses a Pneumatic Brush that cleans the air ducts.  This roto-brush method requires removing all the resisters and inserting the brush down into the air ducts, whereas the air sweep method uses an Air Wand that fits in between the fins of the resister, therefore removing the resisters is unnecessary which saves a lot of time.  The same goes for the main trunk lines, whereas with the roto-brush method requires cutting large holes in the air ducts and the air sweep method only requires drilling tiny holes. 

Unless it has been a very long time (say 10 years) since you have had your air ducts cleaned, most people just cannot justify spending approximately 4x as much money for the incremental benefits of having a more thorough cleaning.  If you tend to have your air ducts cleaned on a regular basis (say every 2 years), it is really unnecessary for a roto-brush method since there really isn't that much build-up of dust yet.