Is $1,300 to clean forced air vents a fair price?

I have forced air heating in a multi-family house, with my own system in the basement. I am concerned because brown dirt is still blowing through my vents 7 days after the service, getting all over my belongings, despite the technician saying it would only blow out for 1-2 hours. I know nothing about HVAC and have tried hanging a cheesecloth on one vent. I initially got this service because I've had horrible allergies for over a month, with itchy, red eyes, and wondered if my heat was causing it. My allergies have improved but are not gone, and I believe the residual dirt is still impacting them. I am seeking advice or help.