The size of a furnace has a dramatic affect on its cost, with pricing rising with the size of the furnace. Furnaces with higher efficiency ratings typically cost more than ones rated lower.
Newer furnaces use relatively expensive metal, and larger furnaces often have more complex interior components. While the price of most appliances scales with their size, the cost of large furnaces scales a bit higher due to the cost of the materials they use.
The efficiency of a furnace also has a dramatic affect on its price. As consumers demand more and more efficient systems, manufactures continue to eke out better efficiency even if the components used increase the price significantly. Efficient units have better insulation to trap as much heat as possible. Specialized components also help ensure optimal combustion, which leads to better fuel utilization and lower heating costs.
A furnace with a slightly higher efficiency rating may cost significantly more than a less efficient unit, but better efficiency often leads to operational savings in a short period of time. Since furnaces stay with homes when they're sold, paying extra for a more efficient unit also adds to the value of a home, so those considering moving at some point may want to view this better efficiency as an investment.