The average cost of a gas furnace in the United States is $2,175. This falls within the $700 to $10,000 installation cost range. The cost of a furnace is determined by its type, size, brand, fuel source, efficiency, home size, and labor. Homeowners can select from three types of furnaces: single-stage, two-stage, and modulating. Single-stage furnaces are generally the least expensive, but they are less efficient than the other two types. Because two-stage furnaces can operate at low or high capacity, they are slightly more efficient than single-stage furnaces. Modulating furnaces are costly to buy and install, but they are extremely efficient and ideal for homeowners seeking more controllability over their home's heating.