How much does a furnace cost for a 1800 square foot house?

Cost may vary depends on the house size, accessibility, the amount of work involved in duct reworking, disconnecting/connecting gas electricity.

Normally, it's better to replace AC/coil/Furnace all together, since a new furnace may not be compatible with old indoor blower /evaporator coil.

The real cost of installing a furnace vary lots of depending on the low end or high end product, accessories (media filter , humidifier) you like to have.