I just found your post and hopefully you have found an answer to your question by now. Just in case, I thought I would give you a couple pointers.
1. Spending more than $400 on duct cleaning is pointless. Many companies do a very good job for less than $200.
2. Duct cleaning should take no more than 2 hours unless you have an unusual system or only one technician shows up. We are also installers and can install new ducts in about 6 hours!
3. CHECK WITH THE BBB!!!
4. Once hired, follow the duct cleaner around and ask questions. People tend to do better work when they are watched. It's just human nature.
Hope this helps. You might want to read this article about hiring an Air Duct Cleaning