Getting the right match of heat pump, ductwork, and other factors is complex. We have about 2500 sq ft in eastern Massachusetts (Zone 4, the green zone, just below the blue zone) and our contractor put in a 5-tone heat pump for about $18K. We saved a lot of money because we were replacing an oil-fueled forced air. If we had to install ductwork or splits, we would have paid a lot more.
Many installers in our area will leave the old furnace in place as a backup in case of extreme temperatures. The layout of our forced-air oil system did not allow that, but one morning in January it hit single digits and the heat pump could not get above 50 until noon. We were freezing in the house, and I rushed out to get a portable heater. It was not pretty, but it was the only problem, and in a week they came and installed backup heating wires that increased the heating capacity, and we haven't had problems since. We didn't have AC before, and I had to adjust the ductwork to get more cool air into the attic where I work, but made the attic livable.
Bottom line for the first 9 months of this year: Electric bill up from $1200 in 2022 to $3000 in 2023; oil bill down from $2700 to $0. Subtract the previous electric bill, and our heat dropped from $2700 to $1800 for 9 months, and we got cooling in the bargain, which we didn't have before. Your experience may differ because a lot depends the old system you're replacing.