If it needs any of the following, I add that on top of the 10K:windows: 250 eachHVAC unit: 3k-5kComplete HVAC including duct work 5k-7kRoof: 5kKitchen Cabinets: 5kMaybe some other stuff than I can't think of right now.My typical REO rehab in my area and the price range that I work in is between 10k and 20k.

My understanding is a very large cost is to open the walls and run the ductwork, if there is none already.

The agent often seemed clueless as to some of the most important things I look at when viewing a property ... evidence of foundation trouble, A/C ductwork that needed replacement due to local code changes, evidence of black mold (not a deal killer, but certainly something to be aware of), and evidence of termite damage.Then I found a deal on a bank owned foreclosure property and when we told the agent what we wanted to offer, they refused because they didn't want to be known as "low ballers" in the market.

First quote was $10k for new everything including duct work (1450 sq ft house), hoping to be able to do for $6k.

Very few houses have central air and hydronic baseboards tend to be the common heat source so often there isn't even existing duct work to add it to.  

This past July I had a replacement 2 ton split unit without duct work and electrical for $3,900.00 in a 3/2 1160 sq ft home.   

Avg winter temp gets around 37-40 degrees The gas heater is shot and the ductwork is torn (located  in tiny crawl space) I already know cats did it.

How old is the duct work?

Assuming this is central A/C, you probably have a dead mouse or something in the duct work.

Once you start adding duct work for heat/ac you'll never have enough height to make it legal unless you dig down then you have to worry about plumbing backing up.