When making the decision you should consider the Annual Fuel Utilization Efficiency (A.F.U.E.). This tells you how much fuel is being used. A furnace today can range from 80% to 97% efficient. The higher the number the less waste, which means a cost savings for you.
You can expect a written Heater Repair vs. Replace Worksheet from your Airtron heating technician that will show you exactly how your furnace is performing.