Hi S.,
Unfortunately new homes have tons of dust, dirt, garbage and other construction debris in them. I think it is a good idea to get it done right away. I've been in this house for 5 years and had them done when we moved in (figured if it helps decrease the dusting all the better). Make sure you go with a good company versus a cheapo one. They need to have strong equipment. Since the initial vac, I've just tried to vacuum the ducts with my sweeper. Pricing all depends on the size of your house and the number of ducts and vents. Plan on several hundred. We used Monster Vac in the past.
When the kids were younger, we ran a humidifier at night in the hall. Colorado air is very dry. I also run HEPA quality air filters in my house that also give out negative ions. Make sure any unit you get does not give off ozone in the process as that is really bad for your lungs. Email me if you want info on the unit we have. I had to get rid of my ionic breeze for that reason. Indoor air pollution is a huge problem - paint, hardwood floors, dust, dirt, and the cleaners we use in our air tight homes.
Good luck. S.