Replaced all duct work when we moved in $1k (we did this)7.

For example, the HVAC unit was replaced 2 years ago along with most of the duct work, there were two pieces of duct work in the back of the crawl space that the inspector noted had light dust and grim on the outside of the duct and suggested it be replaced by a licensed professional.

That should be 6 times that number for a flip.2) By your math you are showing a 41k loss on the house3) Hard to evaluate scope of work with no pictures but if you aren't doing any framing then why are you doing so much duct work?

Here is some speculation , if the house did not have central air , and it was added to an older existing heating system ductwork , that duct work is insufficent for the airflow of a central air system .  

If I have reason to believe there is mold in the ductwork, we'll bring in a company that does duct cleaning with a biocide.It appears the mold in this house is limited to sheetrock and a couple pieces of subfloor, so hopefully it will be as easy as fixing the leak, cutting out the moldy sheetrock/subfloor and replacing it.

(roof- composition shingle on ranch, a/c heatpump needs to be installed with current duct work from furance, 1.5 jack and jill bath, kitchen, 2 bedrooms , its defiantly outdated!

i had one inspection on my first property...he said the attic door was too small for him to go through, and there wasn't enough room in the crawl space due to the ductwork....thanks jackass..350 bucks for him to tell me that a few outlets weren't grounded basically...

They tell me I need new duct work in the basement and also return vents.

They go to your air handler coils, and duct work. 

When the new HVAC system and duct work was installed, some of the joist bays were used as return ducts.