The cost of a furnace can vary widely depending on factors such as the type of furnace (e.g., gas, electric, oil), its efficiency rating, brand, size, and installation requirements. On average, for a mid-efficiency gas furnace with an AFUE rating of around 80% to 85%, you can expect to pay between $2,500 and $4,000 for the furnace itself, excluding installation. High-efficiency gas furnaces with AFUE ratings above 90% typically range from $3,500 to $6,000 or more. Keep in mind that installation costs can vary significantly based on factors like location, complexity, and any additional modifications needed for your HVAC system, so it's crucial to obtain multiple quotes from reputable HVAC contractors to get an accurate estimate for your specific situation.