Are you talking about zoning a single system (1 AC condenser and 1 furnace) or a dual system (2 AC condensers and 2 furnaces)? If you have a 2 level home, a dual system generally is preferred. Prices are regional but start at about $10,000 - $12,000 for two systems, a little less for a single zoned system. Add to this the cost of any ductwork additions/modifications (these are so home-specific, it's not possible to give an estimate).

Yes, two furnaces and 2 A/C units (attic, basement, and outside for 2nd A/C). Plus running all the duct work in the attic to 4 bedrooms and 2 bathrooms. How much is each furnace and each A/C unit about (so minus the ductwork/labor)?

Prices vary greatly depending location. I've had the situation of two better quality, larger HVAC systems cost the same as a smaller, lesser brand HVAC only because of where I was living. Have you considered a heat pump vs. the traditional AC?

What is a heat pump? Where we live would be at the highest end of the price range. But I want to make sure we're not getting snowed.

OK, so you're not actually installing a zoned system... wise decision IMHO. If you're in Boston, New York, San Francisco, Los Angeles, etc, you can expect to pay more. My earier estimates did not include any ductwork, for the reason stated but also wasn't assuming a major, expensive, metropolitan area. You can expect to pay 50% more in any of those cities.

More on heat pumps, they function down to about 28 degrees F, below that it switches to auxiliary heat. So if your climate is so harsh that much of the winter is below that then there won't be much savings. Zone typically means there are dampers in the duct work to divert air into zones.

NYC area. Too cold for a heat pump, I would think. My neighbor just paid 90K to replace her furnace, A/C, and then add a 2nd zone through the attic with another A/C & heat, vented to the upstairs bedrooms. Sounds really high to me, which is why I was asking. I'll be curious about the quotes I get. Thanks everyone.

"My neighbor just paid 90K to replace her furnace, A/C, and then add a 2nd zone through the attic with another A/C & heat, vented to the upstairs bedrooms"

$90,000??? Then there's something going on here we don't know about.

When I lived in a more expensive area a small heat pump (1.5 ton) with duct replacement was around 10k. I know the new form of Freon does drive up the price but I have to agree 90k sounds too high. You need multiple bids, that really is the best way to answer your question.

Looking at the original post, is it common to put HVAC systems in the attic in that harsh a climate?

I just got a quote for 25K for a two-stage furnace, A/C outside and A/C (adding a second zone for cooling) for the second floor). That's better than 90K for sure. But I will still get more bids.