If your non metal ductwork is that dirty you should just have it replaced AND sealed. Also most duct systems are not very well designed. Look for proper sizing and do not go cheap with one or two intakes (return grills) You need them through-out the house. Here in Austin the duct cleaners are carpet cleaners and chimney swifts. (this is who the duct cleaning equipment mfgs call on when the hvac people do not get on board. They DO clean other types of ductwork and we (the hvac guys) have to fix them. I am not aware of any of these non hvac guys cleaning and servicing the actual equipment. Here in Texas it is against the law for them to do so. Treat the entire house as a system, not just the ductwork and /or hvac equipment. Home performance really does work to increase comfort and air quality while reducing operating costs.
If you’re concerned about dust, allergens or mold, it may be time to have your air ducts cleaned. The national average cost for air duct cleaning ranges from $190 to $250. Air duct cleaning costs will vary based on a number of factors such as your location in the country, the age and condition of your air ducts, the number of linear feet to be cleaned, and any repairs your ducts may need. HVAC cleaning companies should clearly outline what is included in their services and at what rate, so always make sure you understand and have a written agreement before work begins. Here are some examples of average air duct cleaning costs:
You should be aware that although a substance may look like mold, a positive determination of whether it is mold or not can be made only by an expert and may require laboratory analysis for final confirmation. For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mold or simply a substance that resembles it.