My A/C company recommended Blackmon Mooring to clean our duct work. I don't recall their cost but if you go to their website, www.blackmonmooring.com, they have information to help and a phone number. I think they price out by the number of vents you have, so count them up before getting quotes.
Blackmon Mooring came in, cleaned the ducts, disinfected the vents (a little extra money), and identified places in the line that needed better sealing (mastic).
Since having them do the work, I've noticed a tremendous reduction in dust and particles and they were professional and courteous. It was worth the expense, in my opinion.