One of the questions about any HVAC system is the amount it costs for the system over its entire lifespan. This may not reduce the ticket price when a furnace needs to be replaced. However, it extends the number of years the unit will be functional.
If you maintain your unit properly, it should last between about 15 to 30 years. According to Home Advisor, the national average costs for replacing a gas furnace is between $2,000 on an extremely low end and over $40,000 at the extremely high. The normal median range is around $5,000.
If the replacement is about $5,000 and the furnace lasts 15 years, it costs you roughly $333 a year, not including operating costs. If that same furnace lasts 30 years, then it costs about $167 a year. That is a significant difference and happens just by maintaining your furnace properly.