I think that the duct cleaning industry is riddled with crooks too.
I was trying to get an estimate in my old house to make my A/C better, including adding more ducts, bigger unit, etc.
One guy came in and would only recommend duct cleaning. A long story about how he has the best vacuum, etc.
HOW can you know that he did a good job? Are they gonna show you the dust? Are you sure it's your dust.
Duct cleaners post signs around the neighborhood. $29.95, etc.
Also, keep in mind that this thing was never heard of before about 20 years ago? Like the sundried tomato

Jacktucky