You've probably seen the ads: $49 for the whole house, $69 whole house unlimited vents, coupons for a duct cleaning cheaper than a takeout dinner. These are almost universally scams.
The pattern: crew shows up, does a 15-minute shop-vac pass at the registers, then finds 'mold' or 'severe contamination' and quotes $800 for what should have been included.
Real source-removal duct cleaning takes 2–4 hours, requires industrial equipment, and has real costs behind it. Anyone advertising an all-in price under about $200 is either running a bait-and-switch or literally not doing the job.
Ask three questions before you book: (1) Do you use negative-air equipment on the trunk line, or just at the registers? (2) Is the price you're quoting the price I'll pay? (3) Can you show me before-and-after photos of a job you did last week?