in my case: 3k at most for a replacement gas furnace, and heat pumps would require new wirings as well. Many things would need to break at once to make it worth considering when you have older homes
New homes with mandated PV and battery, that's another story.
Where I live every house has the wiring for AC anyway, so a heat pump only needs a different valve in the system you will always have (this is about $50 in parts, but they charge thousands extra for it)
I paid for it anyway - my furnace was 50 years old (probably 60% efficient when new), and I have no idea how bad the AC was. That said, for less money I added more insulation to my house and that has a better ROI. (unfortunately the design of the house doesn't allow for even more insulation even though I could save a lot more if it could be done)
New homes with mandated PV and battery, that's another story.