Natural gas, single heating zone, 1350 sq ft - but you didn't say what area of the country. Doesn't matter; I'm in western NY, put in a brand new furnace (85% or 90% efficient, can't remember which) less than 2 years ago. Installation took a few hours.
Next to impossible for me to purchase on my own. Heating contractors wouldn't sell to me, and suppliers won't sell to anyone other than heating contractors. Finally found one contractor that I was able to purchase through. It was around $750 (I probably made a thread on it; I can't remember the exact price any more.) In order to get the warranty on it, I had to have the installation inspected by the heating contractor I purchased it through; he did so for $50 or $100; something like that, definitely no more than $100. When I picked up the furnace at the supplier, on a Saturday, I had to write out the check to the contractor I purchased it through; and his account was billed for the furnace. So, I don't know how much of a profit he made on the furnace itself.
The price to the heating contractor from the supplier varies, depending on the volume of business that they do. When my bro-in-law came to help me install it, he told me that he could have gotten it on his account through a company he used to work for; "probably for about the same price" I managed to get it for, which he said was a pretty good price the company he used to work for, and the contractor I got it through are two of the largest volume companies in the area.
I'd hope that $3500 to $4000 quoted above includes installation of the duct work or something, because to take my own furnace out & do the sheet metal work to make the new furnace fit in, took only a couple of hours. Installation was a piece of cake, but it really helped a lot that my bro-in-law knew what he was doing - by myself, it would have taken more than twice as long, since I would have had a lot more to figure out. I did the gas line connection & new electrical connections while he was fabricating all the sheet metal.
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