There are a lot of variables to provide a short sweet answer, but the best one I can give is, unless you can afford a high production truck (and have enough business to support it), it isn't worth it. Your time is better spent building your carpet cleaning business and expanding that. I am basing this on using a portable, electric negative air cleaning system, not a brush attached to a vacuum hose shoved down a customer's ductwork from the opening. Duct cleaning is harder, nastier and more laborious than carpet cleaning. Labor is the most expensive cost in a business. I initially ventured into it years ago when all we did was primarily carpet cleaning. I think I was bored with simply carpet cleaning. I convinced myself it was good money, but over the years and as we our business evolved to a restoration company, I now see that it is not nearly as profitable as carpet cleaning. We do it now as part of our restoration process for smoke, fire and mold remediation, but if I had it to do over again as an add on to carpet cleaning, I wouldn't touch it.