answer:When I got a new furnace nearly two decades ago, it cost 2k. I think they’ve doubled or tripled in price since then. Maybe 5–7K. Then you may have to dig up a trench to put in natural gas. That might cost 1–2K, depending on what you’ve got to go through. Oh, you’ve got to take our the old furnace. Depending on access and size of tank and whatnot, that could be $500 to 1000. $6,500 to 10K. Pretty much the same as you. If there’s already gas in the house, it wouldn’t be as expensive. Gotchas: well, the furnace tank might have to be cut up to take it out if it’s in the basement. There could be environmental issues if the tank is outside. Make sure the furnace can fit through the access to the basement. Make sure it is large enough for the house. You can either get it big enough to heat the house on the coldest day, or not quite that large. You’ll be cold—a little—on the coldest days, but you save a little money on the cost of the furnace.